Anulpha Pass is the easiest to me. Sebenco and Chenghou are the hardest. I'm speaking of forward directions only, haven't done the reverses too much yet, still working on re-defeating the grid at Elite and earning the various other trophies (guess I'll get to the reverses when Arcade Perfect comes about).

I should add that Vineta K is my least favorite, and my brother's as well. Something isn't right about it.