Blooper is pretty fun. It's useless with real people, but with computers, they suddenly turn retarded when it's used on them, allowing you to zip right by.
as you didn't specify which mario kart game and I've only played N64 and Gamecube versions, I have to say I like the giant koopa shell of Double Dash...
I Didnt ask to be Secretary of Balloon Doggies, the Balloon Doggies demanded it!