Super Mario Kart is where the series started, and was at it's best, maybe it's because this one was more original for it's time, maybe it's because it's just allot better. But simply: it's allot more fun.
The amount of levels in this game is huge, and even though most of them look the same, you will easily notice the difference in levels while playing. The game requires allot more skill then the rest of the series, and i seriously mean ALLOT more skill. It really matters if you know all the fastest ways trough the levels, instead of just getting the best items.
Super Mario Kart will probably remain the best of the series, because the game is less about luck then the rest, wich makes the game more fair, and fun to play.