Well that’s kind of a big question, and since I don’t know the types of games you’re really into, I can just kind of suggest several of the best across the genres.
For action adventure, it doesn’t get any better than the two Wii Zelda titles, both fantastic. Twilight Princess and the recently released Skyward Sword. Keep in mind, Skyward Sword requires Wii Motion Plus. Another sleeper title that didn’t get a lot of hype was “Okami”, and it’s definitely a must-play in the adventure genre, such a great game. Star Wars: The Force Unleashed is great fun with Wii controls too.
Of course for platformers, you can’t go wrong with New Super Mario Bros. A couple of other great ones include Kirby’s Epic Yarn and the just-released Rayman Origins.
If you like strategy games, (with a hint of RPG), you might enjoy Fire Emblem: Radiant Dawn (or the very good previous gamecube title Fire Emblem: Path of Radiance, but a GC controller and memory card are needed)
There’s not much in the way of Shooter titles, but Metroid Prime 3 Corruption is a solid game, especially for a metroid fan. (Avoid ‘Metroid: other M’) You might enjoy the hyped-up “Conduit”, but I would go for the new 007 Goldeneye instead, or the really fun rail shooter House of the Dead, Overkill for that matter.
As far as arcade style games, you could try Lego Star Wars III: The Clone Wars, one of the best star wars games and lego games out there. Lego Harry Potter is pretty good too.
I wouldn’t really recommend any sports, fighting or racing titles since you’re probably playing by yourself, but of course Mario Kart Wii and Smash Bros. Brawl are among the greats in those categories.
If you wanted to elaborate on the types of games you like to play I could probably narrow it down a bit more, but that’s a good list to get you started no matter what.