I'll pick from each generation.
SNES: Gotta love the disco-dancefloor Rainbow Road. The original treacherous course to race on.
N64: Moo-Moo Farm, Rainbow Road
GBA: This one had the least fun tracks, in my opinion. They all felt like rehashes from the SNES days.....but Sky Garden was a good one.
GCN: Wario Colosseum, Rainbow Road, Mushroom City (not sure on the name of the last one)
DS: Waluigi Pinball, Tick Tock Clock