I have to go with Baharroth on this one. He's not a super-uber deathdealing machine, but who else can deepstrike anywhere on the board multiple times, and with a jump pack too? Despite his comparatively meager equipment I've seen him carve up GK Terminators, whole 20 man Wych squads, and lots of other stuff. I guess I just have a lot of luck with him. Usually I send him after isolated targets like lurking Battlesuits or Devastators. He makes a great assassin of units weaker than him and if he gets into a bad situation he can always fly away and come back somewhere else. Plus he's the best looking Phoenix Lord of all.