Mine is Delfino Square from Mario Kart DS. I love the Sunshine nostalgia (even though it doesn't resemble Delfino Plaza in the least) and the creative shortcuts. I can't wait to try that one out in Mario Kart Wii.
I hope you guys are excluding battle mode (coughBlockFortcough)
Er. I like Waluigi Pinball and Delfino Square from the DS,
Koopa Beach and Mushroom Kingdom from the 64,
Toad Bridge(?) from the GCN, and
Luigi Circuit(?) and Ghost Valley from GBA.
Haven't played much of the SNES version. Overall, I'd say Delfino Square is my favorite. (Snakey snakey!)
the games have so many good tracks it gets kinda hard to choose, but i'd have to say
its probably a tie between bowsers castle on the n64 and rainbow road on the wii
Rainbow Road! <33 Though I always get my bum kicked in the Wii version on that track. I'm terribad at it. XD But still my favorite! If I can conquer it, I think I'll be more of a force to be reckoned with in that game. As it stands now though... No. Just no.