For a well rounded group in all four roles, and 3 different power sources (Arcane, Martial, 2 Primal):
Striker: Rogue, DEX/CHA build. Skills: Acrobatics, Bluff, Intimidate, Stealth, Streetwise, Thievery.
Leader: Bard, CHA/INT build. Skills: Arcana, Diplomacy, History, Religion, Heal.
Controller: Druid, WIS/CON build. Skills: Endurance, Heal, Insight, Nature.
Defender: Warden, STR/WIS build. Skills: Athletics, Dungeoneering, Nature, Perception.
Result: All 17 skills covered, with some overlap on Heal (with the Druid being the true person for the Heal check, the Bard picking it up for emergencies despite not having WIS primary/secondary), and Nature (shared between Druid/Warden.) Hits all four roles. And likely has a bit of fun in the process. ;)
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
In my experience, the Controller role is easiest to discard. With the later Player's Handbooks, though, there's more classes that fill a primary role and a secondary role, so any role is up for grabs for discarding.
In smaller groups, I find that choosing a theme for the group - maybe based off power source - can be fun, and then you can build out the roles from there. A particularly fun group can be made of Primal characters. Druids (Controller), Shamans (Leader), and Wardens (Defender) can all be built with Striker as a secondary role, which should fill that nicely. A Cleric (Leader), Paladin (Defender), and Avenger (Striker) party seems like a fun opportunity for righteous smackdowns and roleplaying opportunities.