Out of these four games, which 2 are the best?

For me it's Mario Kart 64 and Mario Kart DS.

It depends really. Diddy Kong Racing is a far better single player experience than Mario Kart 64 but Mario Kart 64 owns it when it comes to multi-player.

Mario Kart DS owns the other three games anyway.

I'm guessing your 2 are Diddy Kong Racing and Mario Kart DS?

Diddy Kong Racing beats the pants off the Mario Kart games in regards to single-player (it has a big island to explore, challenges to complete, boss races, and an overarching story), but MK owns the multiplayer arena (especially MKDS).

Mario Kart just has much-better-designed tracks, and an actually-fun Battle mode.

 Because it's devilishly good...

Yeah, sounds good to me. =]

Mario Kart ds,I still need to get diddy kong racing ds.

diddy kong racing on N64 and mario kart DS, i bought diddy kong and mario 64 with my N64 and i didn't need another game for months, i don't have fond memories of mario kart 64

I haven't played Diddy Kong Racing DS, but Mario Kart 64 is one of my favorite games ever. Mario Kart DS is one of the best DS games I own, so I have to go with the Karts. Diddy Kong 64 just got really old really fast for me, but it was still good enough for me to want the DS version. I just haven't gotten it yet.

Mario Kart DS and Diddy Kong Racing 64. Like others have said, MKDS is much better in multiplayer and Diddy Kong Racing in single player.