To me the must get games for the Wii are: Super Mario Galaxies, Metroid Prime Corruption, Resident Evil 4 (if you haven't played it yet), Battalion Wars 2, Super Paper Mario, Madden NFL 08 & Trauma Center.
There are also loads of games worth checking out on the virtual console but if you're a collector type of person I'd stand clear from it because the prices are a bit steep.