I really like Wii Rainbow Road, but I still think Mount Wario is my favorite track. It feels like a journey, jumping out of a plane to the peak of a mountain, then proceeding though a cave, a forest, and finally an Olympic-like descent before finishing with a full crowd surrounding you.
I like a large majority of courses in Mario Kart 8, but my favorite has always been Music Park/Melody Motorway. It's so charming with its details and just feels great to play. Even though it isn't as ambitious as some others like Mount Wario, it never fails to make me smile.