Heavy. The faster karts & bikes respond better imo, even if they look silly being all fat. Also because the smaller ones get knocked around and fly around the track a lot more in the more modern games than they did in the older games. Or so it feels anyway.
I like using lights, as they do have good acceleration, but it's pretty annoying when you accidentally knock into Bowser and you're sent flying off the track.
I'll be using Roy as my heavyweight when I unlock him.