Honestly, I believe the three best games for the system are the first party titles. Wii Sports, Zelda, and Excite Truck.
Beyond that, the ones I think are possibly worth owning. Trauma Center (if you're into that), Monkey Ball (if you're into that series), and Rayman (if you just want some fun minigames).
Other than that, I wouldn't spend any money on the others myself.
I will be all over Metal Slug next week though.
Beyond that Elebits might be good in December, as well as Super Swing Golf. January has WarioWare, and Wii PLAY.