I still love the original (I think you had to play it in its time to appreciate it).
Otherwise, here's my list:
1. Mario Kart 64 (probably on the short list of greatest split screen multiplayer games of all time)
2. Mario Kart 8 (if this had even a slightly decent battle mode it'd easily be the best)
3. Super Mario Kart (nostalgia and amazing concept for the time)
4. Mario Kart Double Dash (I think this is waaay more fun than it gets credit for)