Wario's Shipyard, then again I'm always going to be biased towards stages that actually take stuff from Wario's games or at least based off of it, but even without that I think it's a cool track.
Mount Wario and Wario Colloseum both fall into the category ''great track, even if they really don't have anything to do with Wario.''
Wario's Gold Mine is fun and fits his character, plus the MK8 remake is one of my favorite tracks in the game, from a visual standpoint.
I actually didn't care all that much for the DS Wario Stadium in it's original form, but the MK8 remake is very nice and N64 Wario Stadium is okay, nothing outstanding, but not annoying or anything either, not to mention it's fun to screw over opponents at that particular jump near the end.