Lego Star Wars has a ton of content, and can be played co-op. Guitar Hero, Tiger Woods, Alien Syndrome, Ghost Squad, RE:UC, and Geometry Wars have the most replay value out of the games I own. Puzzle Quest has a ton of reply value too (I own the DS version). Link's Crossbow Training is pretty good for replay too, and I would play it more if I didn't have Ghost Squad and RE:UC.
I also played Alien Syndrome for about three months straight with a friend in co-op mode. If you can get past the PSP-like graphics, then I would suggest getting that game. It has a leveling system for your character (lv. 99 max), and has skill, and weapon systems that are both upgradeable. If you've ever played Diablo, or any of the Diablo clones around, that's what this game pretty much is.