the most difficult is Wii U Rainbow Road because of 200cc:
The first turn is a guaranteed way to fall off; a high Miniturbo stat causes your UMT(purple Miniturbo) to charge too quickly, causing you to hit the second turn with too much speed(you can't Brakedrift)
the only way around is to charge a SMT(orange Miniturbo), and hop to align. If it doesn't, try lowering your Miniturbo