Infernal Warlock + Tiefling + Hellfire Blood + Flame Wand + Gloves of Piercing (to break that pesky resist at least once per day) = Flamey goodness.
There are an awful lot of blast and burst effects that suplement the usual warlock role of striker, as well.
Give him more hit points for the high constitution, temporary hit points from his pact boon, the tiefling's ferocious rebuke with the push 1 effect, a very high INT score for his status effects, reflex, and armor class, and he can be one very mean machine.
You'll definitely feel the effects of being a one-trick pony when you face fire-based creatures, but the natural fire resist and at least decent non-fire attacks can ride those out, too.
So yeah... I'm pretty satisfied with infernal the few times I've tested it out.
Our primary party's star lock is a touch anemic by comparison. The fey lock we used to have in the party is something of a bad joke. Never tested dark, though.