Rayman Origins is a good game with 4-player co-op play. I can't say much for the quality of the co-op per say as I've only played it alone, but the game itself is very fun with a beautiful visual style, addictive platforming, and catchy music. I will say though that difficulty sneaks up on you so there are chances that you will die... Alot. It can get a little frustrating at times yet I usually still found myself playing through the hard ones until I got them done(the game also will offer to let you skip if you die a certain amount of times if you end up just wanting to get to the next level).
Kirby's Epic Yarn is another good one, this one only has 2 player co-op. It's difficulty curve is a whole lot milder by comparison. It's still pretty fun though with another really nice and unique art style.
Kirby's Return to Dreamland is another, I can't say much for the game quality since I haven't had a chance to play it quite yet. But it does offer up to four player co-op and I believe you have your choice of characters in that one. Even if someone already picked the same character(I know you can at least have two Kirbys).
Edit: Oh yeah and if you don't happen to have friends around when you feel the urge to play all these also can be played alone.