I'll agree with everyone who recommended Super Smash Brothers. It looks like a killer game.
Also, Mario Kart Wii is due out in January and will be a must-have.
My lineup is small, but has enough for all of us.
Legend of Zelda: Twilight Princess. Absolutely awesome game. Lots of fun and variety of challenges. If you're a Zelda nut, you have to have it.
Rayman Raving Rabids. Good choice. It's made up of something like 80+ mini-games and some are downright hard. So it's good for kids and adults.
Medal of Honor: Vanguard. Well...just leave it on the shelf. If you're into WWII FPS's, I'd wait for something better.
Cooking Mama Cookoff. Wow, that's a weird one. But fun. It's great for kids. And kinda sorta shows them how to cook. It's a bit tough but weirdly fun.
Metroid Prime 3: Corruption. I'm in the middle of this one now. No doubt the best Wii game I've played. Just a hair better than Zelda.
And for a freebie, you'll be totally addicted to Wii Sports. It is pretty simple, but ya just can't put it down!