I'd say Twilight Princess.
Or Resident Evil 4. I've not played it on the Wii, but it was great on the GameCube, so I'm assuming that it'll be pretty good.
And all these people, I've noticed, are Pokéfans. For lack of a better word, almost obsessed with Pokémon. So if you don't like Pokémon, I suggest not getting that game just yet.
And DO NOT get Mario Party 8. It sucks.