Mario Strikers Charged is the single most underrated game of all time. I know someone who runs a site for it and it's a great game overall. Only problem is, there is a recently discovered glitch in the game that practically ruins all competition if used, but if you can find players who don't use it on a website (or even on ranked play if you're fortunate), it's a very deep game that beats every wifi game out there right now, both in terms of gameplay and online capabilities.
Call of Duty 5 is also the best shooter on the Wii right now, and is extremely fun, although it's nowhere near the quality of any good FPS due to the lag, weapon imbalance, and uncanny ability to support nooby styles of play. Still, it's a must have for any Wii owner, like Strikers.
Those two should be at the top of your list if you're considering buying a Wii game.