No More Heroes. It's...kind of hard to describe, but if you've played SUDA51's other game Killer7 for the GameCube (and you should), you'll get the idea. Kind of a surrealistic parody of games in general, slathered with a ton of lolwut dialogue and other absurdities. The man's games are pretty much like nothing else out there, and definitely worth a playthrough.
Further on down the pike, there's a very unique title in the works by Warren Spector that goes by the name of Epic Mickey. Take the familiar cast of Disney characters, mix in a liberal helping of steampunk, throw in an animated world gone awry, and make the whole thing
dark, and you start to get the idea. The early concept art and interviews about this have a lot of people excited, and I for one can't wait to learn more about it.
Oh, and the usual standbys of Super Mario Galaxy, Twilight Princess, Metroid Prime 3, and Mario Kart Wii are all necessary fare, though I sadly don't yet have any of them except for the last myself. The Wii edition of Resident Evil 4 is another must-buy if you've never played that, though maybe not as much if you already have it for GC or PS2. As far as anything else goes, my lack of a job means I honestly haven't played much for the system, but anything near the top of
this list should be good stuff. (Note that there are some WiiWare/Virtual Console titles mixed in there.) There's apparently quite a bit of good stuff coming down the pike for the Wii this upcoming year, though I'm a bit fuzzy on details; poking around GameSpot or the other usual news sites should give you an idea of what to look for.