The best single-cart multiplayer game by far is Mario Party DS. Well, you may or may not like that type of game, but you have to at least admit that having full download play (not just minigames, or some subset of the game, but the entire game), and having it work so incredibly well, is pretty darn fantastic. Plus, it is a good Mario Party game. I definitely recommend it!
I also like the primary multiplayer game in New Super Mario Bros. I haven't played the other multiplayer games much, so there may be some good ones among those too - I don't know.
The most disappointing single-cart multiplayer game that hasn't been released yet? Kirby Super Star Ultra. I was really hoping they'd manage to do something like Mario Party DS, but apparently they haven't. You need two copies of the game to play the real game multiplayer. All you get via download play are some mini-games. That instantly makes it "Kirby Super Star Lesser Version".