I think it depends on the kart you use. ive fully completed the game so i can use any kart. If it does depend on the kart, then which kart is the best for drifting?
Dry Bones in the Dry Bomber is what I use. Not sure if that's the best for drifting, but I do know it's the highest acceleration and handling I've seen of any combination of character & kart.
i think that the best person to drift with for me is mario he has realy good controling on his carts and isint that slow in the corses but that is just what i think