It's been so long since I played Mario Kart.
Favorite course overall? I'm going to go with Waluigi Pinball. It had a unique level design with bright and flashy visuals. You've already got me hooked if you have those aspects.
I know everyone hates it with a passion, but Mario Kart Wii's Rainbow Road is my favorite. Sure, it could be a pain, but the music and visuals of the stage sure made up for that.