Part of me wants to suggest SSX(or SSX Tricky) for the casual gamers, but they're actually pretty difficult games in my opinion. Maximo is a pretty easy game to get ahold of I think, and it's entertaining enough that they'll actually want to. Mister Mosquito is great as well. Rez is a game that pretty much anyone can play well, and it's a lot of fun.
I enjoy playing Kengo against my friends quite a bit. Mainly because we yell and do cheezy fake dubbing when we talk trash. NBA Street is fun to play with friends, especially if you don't feel like playing a simulation. Dead Or Alive is a good fighting game to play, it's great for casual gamers (in my opinion) because pretty much anyone can pull off some great moves in it.