They're all good and bad in their own ways. I'm going to rank them.
1. Mario Kart: Double Dash!!
+Pros: Excellent multiplayer, fun tracks and modes, co-op mode, 2 racers per kart.
-Cons: Steering can be pretty off, some of the courses are really short.
2. Mario Kart Wii
+Pros: Excellent multiplayer, wealth of modes and content, online play.
-Cons: Graphics look barely any better than Double Dash, 12 racers can get chaotic, the AI is rubber-banding and seems to get every item when they really need it.
3. Mario Kart DS
+Pros: Online play, great modes and content
-Cons: Some courses are really short, Controls are sometimes a little off.
4. Mario Kart 64
+Pros: Great courses, fun battle mode, good multiplayer.
-Cons: Mediocre graphics (c'mon, sprites?!), poor sounds (except the music).
5. Super Mario Kart
+Pros: Great music and gameplay
-Cons: Stiff controls, steep learning curve
6. Mario Kart: Super Circuit
+Pros: Great music and content
-Cons: Stiff controls and steep learning curve.