Well if you care for older arcade style beat-em up games I'd say Dungeons and Dragons Chronicles of Mystara is a good choice, both Tower of Doom and Shadow Over Mystara are pretty good and they are clearly balanced for multiplayer, if you play them solo it's nearly impossible to not use a shit-ton of credits and if you're playing co-op it's actually do-able without dying but still pretty hard, you need to be well coordinated and be sure to look out for your team-mates, prioritize well who the Cleric should heal and so on, it's surprisingly fair for an arcade game, of course there are a couple of bosses that one hit kill you but if you are organized you can get them without dying, of course you need the right kind of players to do that thing but it's good.
Another obvious choice should be Trine 1 & 2, fantastic 3-player co-op puzzle platformer that actually features multiple ways to get through puzzles, it's damned good fun, specially since every character is completely different and in co-op you have the additional challenge of having to make all 3 characters get through the levels[footnote]A lot of puzzles can only get one or 2 characters through, without some creativity[/footnote] which is actually like a bonus puzzle, I'd say probably one of the best co-op experiences that exists.
On that note Battleblock Theater is also a damned good co-op platformer that's worth a try, it's also really cheap, sure it's not as good as something like Trine but it does enough to be worth it.
Metal Slug is also always fucking good in co-op, of course it's only 2-player but it's Metal Slug and it's awesome, seriously though no one is going to be disappointed with Metal Slug, in fact I suggest marathon every single Metal Slug one after the other, now that's the right way to do it, Contra is also good for this but in Contra you have to be a lot more well coordinated than in Metal Slug.
And finally if you care for older ARPGs I would highly recommend Nox, it only has 3 classes but it's good, it's like just in that sweet-spot right in the middle of Diablo and Gauntlet and it is damned good, I would highly recommend that.