Hmm... I would have to say Sol II reverse is the hardest for me. Really, I suck terribly on all of the reverse tracks because I usually only race forward.
Chenghou Project is actually one of my favorite tracks. I dislike sharp left, but other than that the rest of the track is great. Maybe I am just a sucker for large jumps. The reverse, however is very difficult for me.
Sebenco Climb is also a pretty difficult track, but as long as I don't have to do the reverse track I am fine with it. I still won't race either on phantom.
Sol II is somewhat brutal for me. I can race the forward, but pretty much wall grind through most of the reverse. I also tend to fall off the track when doing the reverse as well (hence my vote).
I like Modesto Heights. I bounce around the inside a lot, but as long as I can get in front of the pack (or if I am doing a speed run), I don't mind it. It only becomes a problem when you are in the middle of a pack of ships and bounce between them and the wall (fighting for the center).