The characters don't matter anymore! You can use any kart/character combination you want! Wahahahahahaha!
I like Yoshi best, because I used the Egg 1 for most of the GPs. Toad's 2nd best, I've always preferred the little guys... I don't know about R.O.B., his first kart is weird. The standard RB is good, but Bowser in the ROB-LGS is a little disturbing. Actually, Bowser looks all wrong in any kart smaller than one of legs would be. R.O.B's good at staying off my nerves, with just a little mechanical whirring, but... he's still a bit weird. Maybe Yoshi55 could explain the ROB-BLS is supposed to be.