I got insanely good at Mario Kart Wii. Super heavy on the Flame Runner motorcycle is literally the only way to play. Hardest turns, highest speed. I did beat all the hard ghosts too with their own Karts, was fun.
That aside, Double Dash co op is ridiculously fun, adds a whole extra layer to the race with the punches, faster spark generation for boosts, someone only focusing on laying down punishment, not even including the fact it's the first one with kart stats, weight classes, and, AFAIK, the only one that had character specific items. I'd say it's the best one, but that may partly be nostalgia.
The best Mario Kart 8 tracks are the sprints, the F-Zero recreations especially. 200cc is a fun addition, going TOO FAST is really fun. Crank up the Inital D soundtrack and it's a blast.