If you're anywhere but Australia, Medal of Honor Heroes 2 is the best shooter game, as well as the game that best utilizes the Wiimote, thus far. Only get it if you have the ability to play online (and are anywhere but Australia); the single player is bad. The rest of the Wii first person shooter games are all crap, so avoid them.
Resident Evil: Umbrella Chronicles and Resident Evil 4 are also two great shooter games.
There is the Wii Zapper shell (as well as some offbrand knock offs), but the general concesus is that they're all crap and that you're better off just using the Wiimote without the gun shells.
Finally, although it does not utilize the Wii controls terribly well, the best game for the Wii that will be released this year is being released tonight at midnight at many places or otherwise tomorrow: Super Smash Brothers Brawl. I'd get a Classic Controller along with it. But I'd ONLY get it if you either have people locally to play it with you or if you can play online.