Metroid Prime Trilogy and Okami are the must haves of the console.
You should also get Mario Galaxy 1 & 2 (It's more than just an expansion and better than other yearly rehashed games), also, if you haven't played Resident Evil 4, you should get it too, it surprisingly plays rather well with the Wiimote + Nunchuck, but it also has the option to play it with a classic controller and the good ol' Cube controller.
Those are essential for any Wii library, but then there are other great games, like No More Heroes 1 & 2 (especially 2), Goldeneye is pretty good too and IMO a thousand times better than the N64 original (belive me, I played the heck out of the original back in the day, but you know why people still regard it as awesome?, because they can't take off those nasty nostalgia glasses), also Donkey Kong Country Returns is great fun.
Also, if you're on the mood for retro action, search the Virtual Console if something strikes your fancy, Chrono Trigger and Final Fantasy III just got released there a few months ago.