I voted Gephyrophobia. I always seem to lag more on it and its just boring and falling a lot....
Have to add an FYI here for you and other folks in a similar situation:
Gephyrophobia is BY FAR the LARGEST Halo PC map there is! It might not LOOK like it to Racers, but there's a HUGE area up on top, and with all the walls of The Pit, etc., that makes for a LOT of object surfaces (actually, they're all surface triangles...)
SO, if you have a graphics card that's
juuust at - or even below - the recommended 256MB size - or not DirectX 9.0c-capable (i.e. 3D Transform and Lighting capable,) you'll be getting GRAPHICS LAG on Gephyrophobia ALL the time!