I don't have A favorite, but my favorites are:
-Waluigi Pinball, just drifting along, 90% of the time I'm drifting.
-Luigi's Mansion, lots of shortcuts, great music, interesting design.
-Rosalina's Ice World, actually mostly because of the huge turn at the start and my luck, since I always get bananas and leave them there and hear the others fall into the abyss...
-Actually, BOTH Rainbow Roads. They aren't as hard as people say, just good drifting skills and you are good.
The former two are my favorite ones and the others are slightly lagging close behind.