I Own
Zelda
Monster 4x4 (don't bother)
Madden 07 (Wii controllers make it fun)
Excite Truck (Wish I got it at the release instead of Zelda it's fun)
If you take the Wii to parties then grab Super Monkey Ball Banana Blitz the minigames are a blast with other people.
Trauma Center is a blast but can be beat in 12 hours (might want to rent it)
Rayman is awesome in single player mode, but again you can beat the story in under 5 hours (might want to rent it) Haven't tried multi-player but it's only 2 people so haven't done multiplayer with it yet.
Red Steel which I'm probably buying in the next week or so is fun, you'll learn how FPS are on the Wii and I liked the Multiplayer but there only appear to be 4 maps to play on)
Tony Hawk is more racing down a hill than freestyle, COD3 is fun but not sure it's a must have, Super Swing Golf blows (or so 4 of us agreed after we all ended up in the exact same spot on the fairway after 4 identical drives)