I voted Warlock. It's not the most Uber in combat (I think a heavy ranger wins that contest), but Warlocks are just cool.
They get fun, useful skills like Arcana, Bluff, Intimidate, Thievery, and Streetwise.
They get to zap foes with flavorful powers, for big damage.
They can curse enemies, and have pacts that trigger cool effects when a cursed creature dies. Meaning, they're one of the only classes that can have cool things happen to them even when it's not their turn.
They get decent weapons, decent hit points, and leather armor.
They gain concealment any time they move more than two squares, giving (essentially) +2 to defenses and letting them be decent scouts (regardless of class) if they spend a feat on Stealth. Also, if they invest feats in multiclassing into rogue or especially ranger, they can get some cool powers that let them attack and shift more than 2 scquares.
Those are a bunch of mechanical benefits, but really I just think Warlocks are *cool*.
