Yoshi all the way. Who couldn't love those adorable noises he makes. Plus he is fast and maneuverable. While the big guys may have the top speed when you always have the blue shell after you, you need the acceleration to keep that lead.
Looking back I was so terrible to my friends in that game. When I would get a shell I would slow down in certain parts so I could knock them off the path (ex: wario's stadium, choco mountain). Then continue on to the goal as they cursed my name. Mmmhh, I need to get new friends who will actually play mario kart with me again.