New Super Mario Bros. Wii
This game is crazy-fun. I've been playing it with a co-worker and a buddy and his wife, and with four players it is both A) impossible to get anything accomplished and B) a complete blast. This is like Four Swords Adventures in hyperdrive - you'll be getting fucked over (and fucking other people over) by accident so much, that the intentional revenge trips will seem like nothing.
The chaotic shitstorm of multiplayer is great, which is to say nothing of single-player, since I haven't actually tried it yet. But it's easy to see that with new power-ups (propeller suit!), classic Super Mario level design, ghost houses, Koopa Kids, wall-jumps, triple-jumps, and everything else that's helped make Mario great since 1986 - there's an awesome Mario game in here, too. I'm looking forward to trying that out.
Progress: World 7 (Co-op)