Mambo #5 isn't the hardest, I guess, but It's All Been Done has a few sections that are basically straight downhill speed rushes. The BTO song threw me off at first, because it was the first I'd played with a really bumpy road. The School Rumble first opening, Scramble, turned out to be easier than anticipated, as did Nekohanten from Ranma ½. Motteke! did not disappoint however.
Nothing too much surprised me on the fast end, partially because I was looking for fast songs for the most part, and also just before starting a track, it shows you a brief map of the up and down slopes, and tells you how congested the blocks are on that track.