I'm a fan of MK Wii. It's the online that makes this game, for me. DS is my second favorite.
My least favorite is the N64 version. I liked the new powerslide mechanic and the 4-player but the rubber band AI ruined the game for me. You could screw an opponent so badly that they would go for a jump and end up half a track behind you--and suddenly they will start moving at super speed!! It felt pretty slow, too.
The original (along with SF2--bought them on the same day) was the first Snes game I ever owned. I bought it before I even had the console! It was amazing in its day. I haven't played in since probably 1996, though. Spent so many times perfecting my skill, racing time trials against my brother's ghost data, and fighting my cousin in Battle Mode. To be honest, I haven't enjoyed a version of MK battle mode since the Snes game. Probably due to a lack of people willing to play it with me. Can't say it's the best, though. Gotta go back and see how well its aged.
