Double Dash is my favorite of the bunch.
I don't like the SNES game because the controls are so damn slippery, I've never seen such a cheating-bastard AI system (okay, maybe one other game), and the choppy Mode 7 graphics sometimes gave me headaches. Too its defense, it still have my favorite battle map.
The N64 game improved the controls, but left the same bastard AI in the game. We played a lot of battle mode, but the mp racing was boring.
The GBA is pretty good. The controls and AI are more like the SNES game, but improved. It runs smoother than the SNES game.
I like Double Dash the best because they sorted out the controls, the AI is fair, and the races are little more wild than the other games. The battle maps aren't as good, but the mp racing is actually fun.
I own every MK game, even though the GCN is the only one I really recommend. I'm just a sucker for this series. That's probably because its the most univeraslly appealing multiplayer series around.