Improve Goliath Warblade AC in DND and Pathfinder

Level 1

I think you’re better off taking your first level in Fighter, or an alternative, than Artificer. It’s a difference between more HP and more skills, but I don’t think you’re going to use a lot of skills beyond Craft and Use Magic Device. But I’d probably rather something better than Fighter...

Problems with Fighter

I don’t think Fighter offers very much here. The class is generally quite poor, and only a good idea if there are a series of specific feats you have in mind that you cannot get any other way.

Note that tanking in 3.5 is not easy; the system does not offer very many ways to do it effectively. You cannot simply be capable of taking a lot of damage, you also have to be able to handle a wide variety of magical effects so you cannot get sidelined, and you need to be a real threat in your own right so you don’t get ignored.

Fighter can offer these things with very careful feat selection, but there are better ways. You don’t need proficiency in any particular armor, you’re not using Tower Shields, and Base Attack Bonus and large HD are available on other, better classes. The Fighter’s notoriously weak Will save (that 12 is quite likely to go into Wisdom) mean that spellcasters may be able to very trivially prevent you from protecting anyone.

If you insist on Fighter, though, you can do a lot to salvage the class by using the Dungeoncrasher alternate class feature in Dungeonscape. Get Improved Bull Rush, Shock Trooper (Complete Warrior), and go to town on enemies. If you go this route, you’ll want exactly 6 levels of Fighter. At which point, you’ll qualify for Warforged Juggernaut, which I strongly recommend here.

Alternative Martial Base Classes

Just from Core classes, Barbarian is a very good choice. You cannot use Infusions while Raging, but most melee Artificers are about long-term buffs, which don’t go away when you Rage. If you have Complete Champion, you can get Pounce instead of Fast Movement: getting the ability to move and full-attack is paramount to your ability to actually be threat enough to draw enemies’ fire away from your allies.

Of course, as is usually the case in these discussions, Crusader and Warblade from Tome of Battle are far superior choices. The Crusader, in particular, gets some of the very-few true aggro-controlling abilities in the game.

For completeness, I’ll also point out the Knight from Player’s Handbook II. It’s unfortunately rather underwhelming, but Test of Mettle is one of the other very few aggro-controlling abilities in 3.5. It’s got a lot of limitations, lots of things are immune to it, and the DC depends on Knight levels (of which, most are bad) and Charisma (which Warforged take a penalty to), so I don’t really recommend Knight.

Recommended Prestige Classes

The Warforged Juggernaut from Eberron Campaign Setting is an excellent choice for prestige class, regardless of how you start. The immunities it gives you are great for not getting sidelined too easily. It’s prerequisites are also ideal for a Dungeoncrasher; a Dungeoncrasher Fighter 6/Warforged Juggernaut 10 is a pretty solid, simple build. I wouldn’t bother with Artificer as a Dungeoncrasher, though; you won’t be able to fit in enough levels to make it worth it.

If you do go with a Barbarian/Artificer, you may find Rage interfering with your infusions/magic items. There is no official way around this, but you might ask your DM if you could adapt Rage Mage (Complete Warrior) to allow you to enter with 2nd-level Infusions instead of 2nd-level Spells, and have Spell Rage apply to infusions and magic items instead of spells. It’s a pretty minor change, and Rage Mage is hardly an amazing class, so it might fly. Personally, though, I think it’s unnecessary; simply being careful about when you start to Rage should be enough. Combat Casting is a pretty obnoxious feat tax.

Something Different: Straight Artificer

I’d also seriously consider straight Artificer, since the Warforged substitution levels in Races of Eberron are quite good. Your base HP will be low (but you should have high Constitution), and ¾ BAB will hurt (but probably not as much as you think), but personally I’d want to get Artificer 5 sooner rather than later. Artificers also get pretty solid tanking abilities, since they can get things like wand of shield other, and various survival spells and infusions. Magic items can go a very long way to shoring up the Artificer’s weaknesses (actually, at high levels of optimization, the Artificer is one of the strongest classes in the game, and capable of utterly devastating any of the other classes I’ve mentioned in this answer; that’s non-trivial to accomplish though).

Personal Suggestion

I'd probably dip Barbarian, take Artificer long enough to get some useful abilities, and then go with Warforged Juggernaut. You don’t quite qualify for Juggernaut as a Barbarian 1/Artificer 5, so a dip into Crusader for some maneuvers would be a good idea. Alternatively, I might drop that level into Rage Mage if I could convince my DM to allow the adaptation, but only if I really felt the like Rage was interfering too much; I don’t think it would really.

So I’d plan on something like Barbarian 1/Artificer 5/Crusader 1/Warforged Juggernaut 10 or Barbarian 1/Artificer 5/Rage Mage 1/Warforged Juggernaut 10.

If Fighter needs to be a part, I’d go Fighter 6 with Dungeoncrasher, and never look back: I’d enter Warforged Juggernaut at that point, and if I finished it, I’d find something else to do. Without Dungeoncrasher, I think Fighter levels are largely a waste of time.

For Pathfinder specifically,

YES

As Eugene notes, Alchemical Power Components are your obvious first step - at +1 damage, for a one-time investment of under 50 GP (exact prices vary), you should always make sure to keep one on hand. 2-4 damage.

The Elemental Pupil trait gives you a +1 to all elemental damage with your chosen element, bringing it up to 3-5

The Havoc of the Society Trait adds another flat +1 damage (and Force, rather than Acid or Frost or Whatever damage), so we're now at 4-6.

Now, we still haven't chosen our class yet - and all of the above are valid on any level one character. If we want to maximize our Cantrip damage right out of the gate, Level 1 should be as a Sorcerer with the Crossblooded Archetype. That'll allow us to select two Bloodlines, and get the level one Arcana from both of them. Which is great, because the Orc Bloodline and the Draconic Bloodline offer near-identical bonuses that stack - each will add +1 damage per die (just one for our cantrip) of the spell, for +2 damage. We're now at 6-8 damage at level 1 - a respectable spammable attack!

However, this also marks the point of No Return, because now we've chosen our Cantrip, as the Draconic bloodline restricts it's bonus to damage of a specific element. I'm going to suggest Ray of Frost, though if you're not happy about doing cold damage, you have some options - see below.

Because Ray of Frost is a ray spell, and therefore is 'weapon-like', it should work with Point Blank Shot for an additional +1 damage if fired from within 30 feet. 7-9.

At level 2, we're going to start taking levels of Wizard rather than Sorcerer. 18 of them in fact, eventually, because with the Evoker school specialization, we can add half our wizard level to the damage of our Ray. (If Cold damage isn't really your thing, you can take the Admixture sub-specialization and use whatever element you feel like.) At level 2, that puts us at 8-10, but by level 19, we'll be at 16-18.

As an alternative, we could stick with Sorcerer levels - but be stuck playing a Dwarf - and take their great Favored Class Bonus to add +1 damage to Acid and Earth spells per Sorcerer level. Comparable results to the Wizard option above, but you're swimming uphill against stat penalties, and Acid Splash isn't quite as versatile a spell as Ray of Frost. You can accomplish something similar as a Half-Orc with 1 level of Admixture wizard and 18 levels of Sorcerer, turning Ray of Frost into a Fire spell. (In fact, the Half-Orc comes out one damage ahead I think.)

Now, as we level up, we can pick up a few more feats that will work with Ray of Frost, since, for all intents and purposes, it's a weapon, and feats that work on weapon damage, work for rays.

Arcane Strike is the big one, as it will scale with you, adding +1 to damage at level 1, and for every 5 caster levels, adding another +1 damage.

If you stick with Ray of Frost and Cold Damage, you can do Sorc 1 / Wiz 18, and one level of Witch with the Winter Witch archetype, taking the Frozen Caress hex at level 1, to add 1d4 damage to your Ray of Frost. Bonus, since it's an extra die of damage, it should qualify for a boost from your Bloodlines, adding another +2, or +3-6 damage net. Keep in mind however, that this option is incompatible with Arcane Strike.

There are a few other roads you can go down at this point, to suit your temperament and desired build - just remember, any feat or effect that increases 'weapon damage' will work on a Ray spell, and that's the key here. Flag-Bearer is another notables one that provides a flat +1 to Hit and Damage. (Incidentally, if you can find a way to qualify for Weapon Specialization (Ray) without sinking too much time into Fighter levels, well, that's +2 damage. Similarly, dipping Ranger for Favored Enemy will boost your Cantrip.) You've got a lot of options, but at this point, at a minimum, it's always going to at least be competitive with a mundane weapon making a mundane hit. And it hits Touch AC for good measure.

(Of course you could always go the easy way out, ignore almost all of this advice, and just put a bunch of levels into Rogue. You can Sneak Attack with a Ray spell, and thus, just use Acid Splash against a flat-footed opponent from within 30 feet and do many d6's of damage. But that's kind of cheating. :P)