Voted awhile ago, but
Electrodrome due to it's upbeat tone, with all those Koopas and Shy Guys dancing in the background.
Music is liked as well, and also like the Anti-gravity part where it splits up into a green and pink path, where they both appear on-top one another for a good part of the race. Also, love those Piranha Plants at the end since they're not the ordinary plants - along with their pipes as well, they're coloured perfectly to fit the mood of the area too. Plus - they're dancing when not chomping.
Course layout is good as well.
2nd for me comes
Mount Wario for being a downhill slide from the glacier peaks, to riding off the side of a dam, then into tree/moguls ski areas, and then finally into a ski race zone, complete with ski racing, moguls, and ski jumping, ending at the bottom of another race. And then the characters stop their vehicles as if they're stopping on skis. ;D
It's probably 2nd mostly because of Electrodrome's upbeat tone and the fact that Mount Wario is only 1-lap long (Which I typically hate).
A lot is tied afterwards.