I love Toad's Factory (WII), Toad City (or whatever it's called for N64), and Bowser's Castle (GC)
Koopa Kape and DK Mountain.
DK MOUNTAIN! I Use the Shortcut there!
Mario Kart Wii's version of Rainbow Road is so beautiful that it's my favorite. Otherwise... I'm quite fond of all the Ghost Valleys from the SNES version, perhaps out of nostalgia.
All Rainbow Roads are both cool and challenging, just the way I like it.
Mushroom gorge! I don't know why, it's just really fun :>
The music is incredible for it! Especially MK64 and MK7's music!
Ghost Valley 2, Mario Circuit 3, Koopa Cape, and various Rainbow Roads.