On paper, I'd go with a lot of the choices listed, especially the Barbarian, but in practice I've seen melee rangers really end up excelling in this role, especially if they're willing to drop a feat or two into armor proficiency. The lack of marking is, of course, a problem, but it's offset (perhaps strangely) by some of the ranger's more controller-ey attacks that slow or move enemies. This allows for a decent amount of battlefield control (something both Defenders and Controllers achieve) while maintaining an immediate melee threat (which is something Defenders and Strikers both achieve) without requiring any fiddly multiclassing or peculiar gear combinations.
It does require the tradeoff of ditching dexterity in favor of strength, but that choice is one that any ranger needs to make anyway. The trick is not to think of it as forgoing ranged attacks so much as knowing that you have some secondary range capability to fall back on. As a bonus, in reducing focus on dexterity, the investment in strength and constitution means that you can actually take the heavy armor feats.
Again, not as fiddly and interesting as some other options, but I've seen it at the table twice, and in both cases it's surprised me with its effectiveness.
First, go read the Art of Healing and Know your Role: Leader. Explaining leader-theory is outside the scope of this answer.
Your party, specifically, has a ranged striker, 2 melee strikers, and an impressive melee defender. The barbarian has an excellent MBA (melee basic attack) as does the Warlock.
The best way to mitigate damage is to kill the enemy first. Consider a Watcher Shaman or Skirmisher Warlord to enable melee and ranged attacks from your strikers, based on who needs to kill the enemy the most. If you go shaman, ask the DM to rebalance the spirit's hitpoints so it's not automatically destroyed whenever an enemy so much as glances at it funny. I recommend a 7+level scaling, with the shaman taking 2+level damage when it pops.
Another solid option is the Ardent. Having played every bloody leader in multiple games, I'm having the most fun with Ardent. There are many different builds of ardent, but energizing strike and demoralizing strike will tell your leader what he likes to do.
Your strikers will love you with a glaive based Artificer, though I've found them exceedingly dull to play, even more boring than a ranger, as they magic weapon, every. bloody. turn. Also, adjacency is a lot harder to get than most people realize, especially when the enemies start dropping AoEs.
Avoid sentinel or runepriest (tricky to play, both of them).
A warpriest or cleric could also be effective.
In an entirely subjective stance, I'd recommend Ardent, just because I've had the most fun playing it. You'll enjoy throwing out THP (prevent damage before it arrives!) and whacking people with a greatspear behind the front line. Just follow the handbook and you won't go far wrong.
--Edit--
It depends. Looking at the advice here, a striker heavy party is a trap. It's a very tempting trap, but a trap nonetheless. With warlock, ranger, barbarian, battlemind, and leader, the leader will be busy keeping three strikers upright. Consider instead a party based around one striker, battlemind, two leaders, and a mage (though warlocks can pretend in a pinch...) The striker should have a really nice MBA (melee basic attack) (see: thief, slayer) and the two leaders should be able to trigger that MBA. As a way of making the leaders feel more useful, let them roll the dice on the MBA they grant. This way, you effectively have 3 nasty strikers for the price of one, double-healing, someone who can take care of minions and other zoners.
Looking at your party makeup, here is my recommended party.
- Drow Thief (Dex/Wis)
- Party scout, MBA spammer.
- Later levels: If they grab Two-Fisted Shooter and Drow Fighting Style, quite a lot of fun may be had.
- Longtooth Shifter Skirmishing Warlord
- Direct the strike "Oy, you, over there, thief... go attack!" Both are solid.
- (With a Str/Int race like genasi) tactical warlord + Commander's Strike + Polearm = "Hey, thief, attack again, but do more damage, and I want to roll!" Will do delicious amounts of damage.
- (dex/wis race) Watcher Shaman (replaces the dwarf healing cleric. Wis is required, dwarf isn't bad, actually).
- Claws of the Eagle for "I'd like to have the thief attack again" with the agreement that the leader gets to roll the attack. As she can get spirit's prey (remember to have the thief take drow fighting style), this is 1-2 attacks a turn. Quite fun. Optional is longtooth shifter
- Half-Elf or Tiefling Con/Cha battlemind, party face. Designed to be flanking buddy for the thief, or (more usualy) protecting the squishies while the thief does his thing.
- Conductive Defense and Vicious Cobra Strike are your friends here. Being tough is nice, but you've got 2 leaders, so it's less absolutely critical
- Illusionist Mage, Psion, or other controller to forced-movement enemies into the thief/leaders grinder.
As an offer, if you want to stop by The Back Room with your group, I'm happy to walk you all through collaborative party making. We've tested out a system there and it makes for a very strong RP party that can collaborate well mechanically.
Best Answer
Dwarfs are a Con Str/Wis Race. They have * unbelievably* good feat support for weapons. Looking here, and from my own experience, I can recommend the following:
They fit basically any essentials class, due to melee training and their "Dwarven Weapon Training" feat, a must take if the class wields weapons. They have some interesting melee warlock implications, but are rather more tricky to build. Beyond that, probably the best dwarven strikers are slayers or rageblood barbarisns (in that order.) While ragebloods have better stat synergy, Slayers are far better written, and therefore have more elements that are actually used by a beginning player. Slayers also get scale out of the gate, which means a moderate investment in dex is all that's necessary to get striker damage on everything, rather than AC levels of investment.
From your second paragraph, the virtues you describe more ably complement defenders than strikers, as dwarves' speed does hamper them slightly in the "ability to attack arbitrary targets" criteria. Forced movement isn't a big deal for a striker, since they don't have the adjacency requirements of a defender or ranged character.