Unreal Engine 2: Awesome but expensive
Source Engine: Sweetass graphics, but overall, a crap engine
Quake 4 Engine: Actually, it's really the Doom 3 engine, but wtf. Flawless engine, but Tremulous will loose appeal from people without a) a good computer and b) Quake 4
Quake 3 Engine: Sturdy, flexable, open source
Reasearch before you open your crap hole