Resident Evil 4, Resident Evil Umbrella Chronicles, Mario Galaxy, Paper Mario, Warioware, Metroid Prime 3: Corruption, Zack & Wiki are all worthy of consideration.
Mario Galaxy is THE best game I have played since Vice City, and that is saying something. Utterly well mega. Get it.
Otherwise I'd go with what most other people have suggested. Wario is good if you've got a bunch of mates over, I can't comment on the Sonic & Mario olympics yet but if it's even half as good as Wii Sports it's got to be a winner.
Enjoyed the Mario football jobby too. Forget about Monkey Ball, even if it's going cheap. And download some classics from the Wii shop such as Bomberman and the original Mario Bros.