Because the assassin has access to the Ki focus, enchantment is left to the ki meaning your only selection is in the weapon die and properties you'd like it to have.

Things you should be thinking about

• Weapon die size: how big of a punch do you want to pack, and what is it going to cost you to pack that big of a punch?
• Weapon proficiency Bonus: How much of an impact with taking a +2 weapon over a +3 weapon make on your accuracy (do the DPR calculations and figure it out, don't leave this to hypotheticals).
• Feat/Power support: While a lot of your feat support should focus on the ki focus, if you've got some specific things in mind for a specific weapon group then you should choose weapons in that group.
• Variety: You probably have the ability to quickly switch weapons, maybe you want that Glaive for reach, but sometimes need the accuracy of the rapier or the punch of the flail. This is the kind of versatility granted by a ki focus, take advantage of it.
• Story/Character reasons: think about your character, what kind of weapon do you picture him with? Is he the type to be bristling with blades and weaponry? or is he more subtle and favor easily concealed weapons? Does it change from battle to battle? This is probably the major source of your weapon choices (since most of your special stuff is driven by your ki focus).

By default as an assassin you get access to

• Military heavy and light blades
• Simple one-handed melee
• Simple ranged

There is a pretty extensive list of weapons here, particularly in the heavy and light blades (rapier, falchion, broadsword, glaive, longsword, greatsword). I'd start with grabbing a couple from this list (definitely rapier/long sword and probably a glaive, maybe a greatsword just to have a nice punchy option). Then grab an item or two off the simple one handed melee list, although most of the options are trumped by heavy blade options, however if you've got a story or feat/power issue that would provide a good reason to take something off the simple one handed-melee list, go for it.

Taking a level or two of fighter is a mark of desperation for feats.

Taking more just doesn’t make sense; a feat per level is weak, but if you desperately need feats it can be the right choice. A feat every other level is worthless.

I’m going to answer based on pure fighter, but only after the caveat that I’m taking this as an arbitrary restriction that I would never recommend holding to.

The Defender Role

This is a concept from 4e, where fighters and other Defender classes could meaningfully protect allies and punish enemies for ignoring them.

3.5 does not have any such concept. If you are big, hardy, impossible to hit, with tons of HP, enemies will ignore you and go straight for the real threats. You have almost no way of stopping them from doing it; only magic does that at all well. Tripping is the best bet you have, but it’s not great.

Alternate Class Features, Variant Classes, Substitution Levels

Alternate class features are the only hope here; otherwise, there really is nothing that the fighter has to offer. I suppose you’d get Weapon Supremacy just to say you had.

Dungeoncrasher

This ACF from Dungeonscape replaces your 2nd-level and 6th-level bonus feats with the ability to deal massive damage by slamming people into walls with Bull Rush.

Zhentarim Soldier

These substitution levels from the Champions of Valor web supplement gives you Skill Focus (intimidate) at Fighter 3, extends the duration of the demoralize effect of Intimidate at Fighter 5, and allows you to demoralize as a swift action at Fighter 9. This costs nothing (excepting that you have to keep taking fighter levels, which as discussed is actually a very high cost), and the 9th-level one is actually good.

Between these two, you can almost justify 9 levels of fighter, since the swift-action demoralize is a fairly solid ability. Or, would be if everything and its brother didn’t ignore Fear effects...

Tactics

Fighters have access to three good tricks, more or less. As a pure fighter, you might as well attempt to do all of them, and maybe toss in some other ideas.

Charging

Barbarians do it better, but it is what it is. Wants Power Attack, Shocktrooper (Complete Warrior), Leap Attack (Complete Adventurer), and Battle Jump (Unapproachable East). Really, really wants at least one level of barbarian and warblade, but whatever. Lance is the weapon of choice if you go mounted, otherwise any two-handed weapon is best. Using a lance kind of pigeonholes you into not tripping, so just going with the trip weapons is probably better.

Dungeoncrashing

Only available with the ACF, this mostly relies on Bull Rush, so Power Attack and Improved Bull Rush are necessary. Shocktrooper (Complete Warrior) is crucial for the directional Bull Rushes (you can angle people as you push them), while Knockback (Races of Stone) allows you to Bull Rush on every attack rather than as a standard action. Weapon doesn’t matter for Bull Rush.

Intimidating

Only viable with the Zhentarim Soldier levels, and even then, a disturbingly large selection of foes will outright ignore it. Wants Imperious Command (Drow of the Underdark), as well as the Never Outnumbered skill trick (Complete Scoundrel). Max ranks in Intimidate, don’t completely dump Charisma.

With the Fighter 9 replacement from Zhentarim Soldier, lets you spend a swift action on the first round to attempt to demoralize everyone you threaten, and if successful leaves them cowering for 1 round. Cowering opponents are extremely vulnerable, so this is a really good trick when it works. After the first round, you can continue to use swift actions to demoralize, but only against single targets.

Unfortunately, Imperious Command requires Cha 15. Unless you roll for stats, and roll absurdly well, that’s just not going to happen until very late in the game, because Cha is your least, or at best second-least, important ability score. The only thing it improves is your Intimidate check, which is mostly about your ranks and bonuses from other sources, not your Cha.

Also, Tome of Battle has a new mechanic for Intimidate, called Duel of Wills. It’s a thing you can just choose to do, for free, at the start of combat. Since you’re pumping Intimidate and most don’t, you get some free bonuses, though they are small and fleeting. Still, free.

Initiating

Martial Study and Martial Stance from Tome of Battle are fighter bonus feats. You’re limited to only three of the former, which sucks, but there are some great options.

Obviously, you would be massively better off just taking levels of crusader or warblade.

For maneuvers, shadow walk would be awesome, iron heart surge and white raven tactics are excellent, charging minotaur strike fits in well with dungeoncrashing, etc. Defensive rebuke is one of the very-few true aggro-drawing abilities in the game. There are more. For stances, thicket of blades makes it much harder to get around your attacks of opportunity, and that is really important, because your true purpose here is...

Tripping

The bread and butter of any lockdown build. Combat Expertise, Combat Reflexes, and Improved Trip are necessary; Knock-down and Stand Still are solid choices. You need a weapon with both reach and tripping ability: the guisarme or spiked chain are your best options.

This is how you really lock down a fight. Intimidate’s a nice trick when it works, but it often won’t, and charging and/or dungeoncrashing are usually much more about damage than they are controlling the battlefield.

Tripping means anyone who wants to get past you, has to do it very carefully. You can trip on your attacks of opportunity, which tends to prompt another attack of opportunity when they get up—the AoO happens before they actually get up, though, so trippping at that point has no effect and they still can get up afterwards.

Races

You want to be big and strong, but you don’t want to be stupid because you need that Int 13 for Combat Expertise. Two levels of barbarian would get you around that, and massively improve your life, but oh well. At least it ensures you’ll have skill points for Intimidate and Jump.

Also, when you’re taking levels in a class as weak as fighter, level adjustment may be worthwhile. It’s usually awful, but maybe LA +1 can get us something good...

Goliath

Oh yeah, it can. Goliaths are from Races of Stone, and have both LA +1 and the Powerful Build ability, treating them as a size category larger for a lot of purposes (mostly everything except space and reach, which is a shame but oh well). That means +4 on Bull Rush and Trip checks, and slightly higher damage. The +4 is the important bit.

They also get +4 Strength, −2 Dexterity, and +2 Constitution. The −2 Dexterity definitely does hurt, but it’s rare to make more than 2-3 attacks of opportunity in a round, so you can survive it. The +4 Strength and +2 Constitution are big helps, though the Constitution is misleading because you also have one fewer level to get HP from.

Mountain Movement, the ability to always count as running for jumps, is also useful to you. It will allow you to trigger Leap Attack much more easily. If you can get it, though you probably can’t without a warblade dip, sudden leap would also benefit.

Half-minotaur

From Dragon vol. 313, this LA +1 template makes you actually Large, which means you get space and reach, which are very, very good for improving your threatened area.

Ability scores are Strength +4, Constitution +2, and Intelligence −2. Again, the hit to Intelligence hurts, but probably even less than Dexterity: you just need to make 13. Even if you start with Int 14 and put your +1 there, you can probably work with it.

Since this is a template, it can be applied to any of your other good choices. Going LA +2 as a half-minotaur goliath is interesting, but probably a bad idea.

Dragonborn warforged

Dragonborn is an LA +0 template from Races of the Dragon, which is cool because, hey, no LA. It also replaces the majority of your racial features, with the exceptions of ability score changes, subtypes, and movement modes. The subtype thing is important.

The dragonborn applies +2 Constitution, −2 Dexterity. Again, the Dexterity hurts.

What do we get for it? A few random things, nothing too special, but we get to choose a Draconic Aspect. Heart and Mind are worthless (a weak breath weapon or minor bonuses to select Will saves), but the third option is Wings. This is the correct choice; a fighter tends to have really, really big problems getting into the air, and this straight-up solves that problem.

Meanwhile, the race you choose usually doesn’t matter that much, since you don’t get to keep much of it. The warforged race is an exception, because most of their racial features aren’t features of the race, but rather of their Living Construct subtype, which Dragonborn lets them keep. It’s cheesy as hell, but you’re a freaking single-classed fighter, it’s OK.

Warforged offer a smorgasbord of immunities (poison, sleep, paralysis, disease, nausea, sickened, fatigue, and exhaustion) that will make your life better. It also means you don’t have to eat, drink, or sleep, which is nice. And you get +2 Constitution, while losing on −2 Wisdom, −2 Charisma—two ability scores, but your two least important.

Combined, you’re looking at all those immunities, flight, −2 Dexterity, +4 Constitution, −2 Wisdom, −2 Charisma.

And you could throw half-minotaur on there for +4 Strength, −2 Dexterity, +6 Constitution, −2 Intelligence, −2 Wisdom, −2 Charisma, as well as Large size and some other minor benefits. This is probably your best choice.

Human

Just saying, it’s viable. Think you have more than enough feats? Guess again; you’re going to need everything you can get. True, though, things you can’t get from feats—like size and flight—are more useful.

Conclusion

Half-minotaur dragonborn warforged fighter 20, with the Dungeoncrasher ACF and Zhentarim Soldier substitution levels. The penalties to Dex and Int mean some awkward choices on ability scores; you may hold off on Combat Reflexes until later, when you can get some gloves of dexterity to make better use of it. Late game, even the 36,000 gp for the +6 version is quite affordable.

A 26 PB might be (base) 14 in each of Str, Dex, and Con, and then 15 in Int, while leaving 8 in Wis and Cha. Having −2 to Will saves (and Intimidate checks, but that matters less) is bad here, but you don’t have a lot of option. The solution I’m using will be Steadfast Determination from Player’s Handbook II, to switch Will to using Constitution. At 32 PB, I’d push Cha to 14, for something like this (after racial mods):

• Str 18
• Dex 12
• Con 20
• Int 13
• Wis 6
• Cha 12

Not exactly amazing for LA +1, but we have a lot of requirements. As it is, we’re going to have to stick 1 into Cha from levels, and get a amulet of charisma +2, before we can get Imperious Command.

Other ability score bonuses should go to Strength.

If you are literally stopping at 12th, it may be worthwhile to just dump either Dex or Cha and give up on either Combat Reflexes or Imperious Command. Dumping Cha is the better choice, though it makes less use of your few unique features.

Anyway, we are a Large Monstrous Humanoid, with lots of immunities, and from level 1 can glide. At level 6, that will become flight. The LA does not count towards that, sadly.

We get 3 skill points per level. Intimidate, Jump, and, I don’t know, Handle Animal can be our skills. Cross-class ranks in Balance, Tumble, or Use Magic Device might be better, maybe.

All of your tricks are weak at the beginning of the game; you only get one AoO, your Intimidate ranks are barely beating your penalty to Cha and you can’t get Imperious Command, and dungeoncrash doesn’t really shine until Fighter 6. Still, the Fighter 2 version of dungeoncrash is solid enough, so that’s probably where you start:

1. LA +1, Power Attack

2. Improved Bull Rush

3. Dungeoncrasher I

4. Skill Focus (Intimidate), Endurance—you are literally immune to everything the Endurance feat helps with. See if you can get the DM to waive it as a requirement for Steadfast Determination, it would help massively.

5. Combat Expertise

6. Extended Demoralize, Never Outnumbered (skill trick)

7. Dungeoncrasher II, Steadfast Determination

8. —

9. Improved Trip

10. Swift Demoralize, Imperious Command, absolute last level where fighter gets anything meaningful

11. Knockback

12. —

13. Shock Trooper, Combat Reflexes

14. —

15. Knock-down

16. Martial Study (foehammer)

17. Martial Stance (thicket of blades)

18. —

19. Leap Attack

20. —

Yes, you get nothing at 20th level. Sorry.

For equipment, I recommend:

• +1 guisarme, 4,309 gp—it reaches, it trips, it’s everything you want without burning another feat on a spiked chain.

• masterwork spiked gauntlet, 305 gp—just so you can attack inside your reach, if necessary.

• +1 full plate, 3,650 gp—you have heavy armor proficiency and your Dex is meh. So you want this. Unfortunately, dragonborn flight doesn’t work in heavy armor. Hopefully between a literally-pointless Endurance feat and the tooth of Savnok (see below), your DM can see their way to ignoring that limitation, at least for you. If not, you have to go with a +1 chain shirt (a breastplate is garbage; lots of penalties for just +1 more armored AC—not worth it). Eventually, you will be able to afford +1 mithral full plate, solving the problem.

• tooth of Savnok, 2,000 gp—eliminates the movement penalties from armor. You’re required to never take off any armor you wear, but with the Endurance feat, you never have to. Only relevant if you get wear medium or heavy armor, which really should only be full plate, which as noted you might not be able to use. (Tome of Magic)

• amulet of charisma +2, 4,000 gp—to enable Imperious Command, sadly.

• belt of battle, giant’s strength +2, 16,000 gp—extra actions. Also, bonuses to initiative and Strength. (Magic Item Compendium, both for belt of battle and the rules that allow it to also have +2 enhancement to Strength)

• third eye clarity, 3,000 gp—prevent or remove a lot of nasty effects, including daze and stun. (Magic Item Compendium)

• anklet of translocation, 1,400 gp—quick, cheap mobility. You need it. (Magic Item Compendium)

That’s about 35,000 gp. You have 36,000 gp for 9th-level WBL, so that works out pretty well. Other good options include a belt of healing or scout’s headband. The fearsome armor property from Drow of the Underdark is a bit wasted on you (you already have swift-action intimidate, so the move-action version isn’t that useful), but +5 to intimidate isn’t awful. Getting gloves of dexterity would be nice but you neither have the money nor the feats.

Also, be sure to keep abreast of the usual answers to the typical problems that your Living Construct subtype and/or wings don’t already cover.