It's sad, but I have to say Mario Kart Wii because it's the game I had the most fun with. Every single other Wii game I played was disappointing in some way, if not flat out mediocre altogether.
Super Mario Galaxy, Super Mario Galaxy 2, Fishing Resort, and Animal Crossing: City Folk. Not in any particular order, but if I had to choose just one, it'd be SMG2.
Okami (I'm surprised to see so many Okami fans here, it's not very mainstream popular) and Endless Ocean: Blue World. EO isn't a mainstream game either, it's a scuba diving game and you go do missions while solving a mystery. Sounds funny, but it's a really good game if you like casual games and fish
Legend of Zelda: Twilight Princess was really good. Other than that annoying City in the Sky, which I refer to as Ocarina of Time's Water Temple in the Sky.