A solid Tower Defense, co-op experience depends entirely on how awful of an architect your friends are.

Fun with a friend; fun shooting things- not mutually exclusive.

A fun mini-diablolike, pick it up for the fun of dungeon crawling, stay for the suprisingly in-depth dev kit.

A great 4-player Diablo-like with great character customization, accessible modding, lovely art style, and loot loot loot loot loot loot.

A stealthy game for 4 players, so its good that almost every level has the guns blazing approach. Also has collectible masks.

4-player Frying-Pan Assault and Battery Simulator, now with more TAAAAAAAANK.

Even if it hits dearths of content, Starbound is a fun game to pick up and see what adventures you can have reaching the end game. Building Optional.

For those who like Borderlands and irrationally hate the moon/the word "sequel" in any context. Lots of guns to collect and brag to your friends about.

Badass Ninjas are always a plus, Badass Ninjas with rocket-hammers are a greater plus, and there is a suprising depth of content to experienced players.