While I usually use the regular bike (and only the regular, none of the other) I'll use a kart if anyone wants.
It's funny cuz up until a month ago I used the Standard Bike exclusively. Then in preparation for E3, I switched over to karts, then it immediately hit me: "OMG, the kart enables me to perform decently (and not by luck/chance alone); this is what it's like to FEEL THE ROAD!"
Really, with the Standard Bike I was sliding everywhere, hitting walls, didn't seem very fast, getting tapped by others would bring me to a halt -- i thought that was normal. Turns out I'm meant to use a TANK.