oh god MK64 fans what is this I don't even
SMK is the best single-player experience (haha no blue shell is pretty much an auto-win here), DD the best multiplayer (two players one kart mode is an awesome option, skilled item defence exists).
MKWii is kinda forgettable overall but at least has pretty good track design. MK64 introduced several awful ideas (blue shells, dragged items) and had awful track design.
Haven't really played the portables ones much; blue shells mean singleplayer is only going to be okay and they're poorly-suited to play with a group of friends due to the screen thing. I do hear good things about DS anyway so I'll pick it up some day no doubt.
--
The RPG Duelling League: www.rpgdl.com
An unparalleled source for RPG information and discussion