Let's just put this to bed nice and early. Baby Luigi was, is and always will be the best character in Mario Kart and that's just a straight-up fact. Truth be told, Baby Luigi and the Bullet Bike would rule all motorsport if they were given the chance to do so.
There is no debate, there is no close second, the Baba is the clear winner. First of all, the smaller characters are superior in every way. True racers will agree on that. The likes of Bowser or Donkey Kong are only for those who depend on their size as they try to fight their way through the riff-raff of the mid-field. The real drivers need not worry about that sideshow.