A game not many people know of, but I just got yesterday and is a lot of fun is Fire Emblem: Radiant Dawn. If you're not familiar with the FE series (DS games), it's very similar to the gameplay of Final Fantasy: Tactics. If that's your cup of tea, you'll really enjoy it.
But yeah, other than that, I agree wholeheartedly with everyone who has said these games:
The Legend of Zelda: Twilight Princess - Just like every other Zelda game since 64. It's a lot of fun, and very addicting
Super Mario Galaxy - Again, pretty similar to Mario 64, but with a lot of neat twists and quirks.
Super Smash Brothers Brawl - Stays true to the SSB series and is a ton of fun!
There are a lot more games that cater uniquely to the Wii. The Trauma Center series games on Wii are fun and a unique twist (you're a surgeon operating on patients). I found the Madden games a lot more fun on Wii once you get used to playing with the controls. It makes it difficult to go back to playing on PS3/360 though, because you try to use the accelerometer controls which don't exist
Others I haven't played yet but want to include: Metroid Prime 3: Corruption, Warioware: Smooth Moves, Super Mario Strikers Charged, and others. Need more money (and time)!!
All in all, I'm very happy I made the Wii purchase. It's a lot of fun, and while there weren't a ton of good games out early on, now there are more than enough to keep you occupied!