Here are all the characters in the game and how they rank against each other. They have been divided into their seperate classes and ranked according to speed, cornering, acceleration and off-road capabilities.
A is equal to one point and the higher the score the better they are

THE MIDDLE WEIGHTS:

MARIO LUIGI

SPEED - The middle weights have a top speed that is not quite as fast as the light weights but is just a little bit faster than the heavy weights. CORNERING - They lose more speed around corners than the heavy weights but do not lose as much speed as the light weights. ACCELERATION - They are faster off the start than the heavy weights but not as fast as the light weights. OFF-ROAD - They lose more speed when off road than the light weights but are faster off-road than the heavy weights. TOTAL - 8 GENERAL INFO - You will find eventually that these two characters really serve very little purpose in the game and you will probably end up with a character from one of the other two groups.

THE LIGHT WEIGHTS:

PRINCESS TOAD YOSHI

SPEED - The light weights have a higher top speed than the middle weights and the heavy weights which makes them the fastest group in the bunch. CORNERING - They lose more speed in cornering than the heavy weights and the middle weights but if you are good enough to use miny turbo boosts then you have nothing to worry about. ACCELERATION - They are at the top of the group in acceleration. They can pull ahead of anyone off the green and start off leading the pack. OFF-ROAD - Their light weight stops them from sinking into the dirt when off-road and therefore keep most of their speed making them the best off road. TOTAL - 10 GENERAL INFO - This group is especially good for trying for best times. If there are no other players on the track to mess this group up they are bar none the best.

THE HEAVY WEIGHTS:

DONKY KONG WARIO BOWSER

SPEED - They are the slowest group in the bunch, with the lowest top speed. CORNERING - They have the ability to keep almost all of their speed while cornering even without sliding around the corner. ACCELERATION - They have the lowest acceleration of the group. OFF-ROAD - They are horible off-road because of their weight. They tend to sink into the ground causing their speed to decrease dramatically. TOTAL - 6 GENERAL INFO - This group in my opinion is the most fun. This is because these are the guys you should be using when you are playing against other players. Either in GP, VS. or battle mode, you should use the big boys. Nothing is more fun than smashing into the other karters and causing them to spin out or be knocked off the track. In battle mode you also have the advantage of busting their balloon by simply driving into them at high speed.(always a fun thing)