The Baal Predator. I'd like to say what an awesome force of destruction it is on the table. I'd like to, but I'd be lying. I've had very little luck with it in games, it seems like I never have it in the right place or it's blown up on turn one. I seem to remember a time or two when a penetrating hit managed to take a couple of Marines out as well. Damn explosions!
The 4th ed rules and accompanying Space Marine codex really helped out the Baal. ASSault Cannons, FIRE!! I ditched the heavy flamers for heavy bolters and have been pretty happy. Extra armor is pretty much a 4th ed requirement though.