Buzz, without a doubt. But I don't agree when you put lumines on the list. it's a puzzle game, ok but not a casual game!

Many of those initially listed plus

Endless Ocean
Sam & Max
Mecury Meltdown
Tiger Woods

From that list I'd go with Rock Band and Guitar Hero. =]

I haven't bought any casual games

Mario and Sonic. That game is too amazing in 4 player. I just wish they had done a proper game with 8 player online. 8 player 400m would have been the most amazing thing ever

I'm interpreting "casual" to mean "easy to learn how to play." By this definition, my favorites are...

Wii Sports (Wii)
Rayman Raving Rabbids (Wii)
Guitar Hero (Multi)
Crazy Machines (PC)

Also, I loved Zack and Wiki, but I definitely would not classify it as casual.

Are we using the hardcore gamer definition (= stupid games only idiots would want to play), or the sensible people definition (= games that even non-gamers are capable of enjoying)? =P

Hmm... some of the ones I've enjoyed a lot in the past include Boom Blox and Warioware for the Wii, Tetris on the Gameboy, and all those bloody addictive flash games that keep eating my time. Oh, and Little Big Planet looks great, but I haven't had a chance to try it out yet.

I have bought and enjoyed all NBA Live games starting from 2001, (Live 01 to Live 09) and I immensely enjoy playing it with my nongamer friends. (It helps that we all, also love and play real life basketball)

Mario and Sonic at the Olympic Games

That game is one of the best multiplayer/party games ever. Uses Wii mote in an excellent way plust its tons of fun.

To quote bluesinG

"I'm interpreting "casual" to mean "easy to learn how to play." By this definition, my favorites are..."

"Also, I loved Zack and Wiki, but I definitely would not classify it as casual."

My favorites would include:

Carcassonne (XBLA)
Boom Blox (Wii)
Mario Party 8 (Wii)
Mario Party DS (DS)
Clubhouse Games (DS)