I've been playing a lot of Co Op games for a couple of years now with my girlfriend. We both love RPG-s and Beat'em'Up games. Since i grew up in the 90's i love the old school game style too. Taking the best out of 2D graphics (2,5D) + Nintendo Hard.
I hope you'll enjoy the list:
Castle Crashers (2008)
In my opinion, the king of modern Beat'em'Up games! :D If you have not played this, start with this game, and after a couple of hours, you come back to this list too update your rooster! :D 4 Players with a lot of fun, originality, cool story line, nice skills, pets, mounts,... Just 5/5
Lost Castle (2016)
Awesome Chinese Indie game that went extremely popular. Highly recommend it to everyone. It's Nintendo Hard and playing it feels just like playing a really hard Sega/Nintendo game without the "Save" option, but in a good way. Hours will pass and you won't even notice it, you'll just be happy to see what weapon and skills you'll get next from a massive weapon storage! :D
BattleBlock Theater (2013)
Co-Op game where you have to work with your partner in order to beat the game. If you are a solo player, you'll die! :D
The Bug Butcher (2016)
Have you ever played Pang!? Well, this is a modern version of that game :D Enjoy it.
Move or Die (2016)
Best Partygame Ever! :D With a lot of game mods and an awesome game play for 4 Players, one of the best Party games for the pc ever :D
Viking Squad (2016)
The only game i didn't bought on sale. It's a nice game with a Lane system. Can be finished really fast, and could have some more stages, but not bad :D
Tricky Towers (2016)
It's an awesome Chilling Tetris like game with a couple of game mods. If you start playing it, you'll surely have the feeling like a little time skip happen, because its addictive. :D
Orcs Must Die! 2 (2012)
AWESOME Tower Defense / RPG game. I personally love it :D True, there is a newer Version of the game Online now and it's free to play, but this one is epic and made for Lan Parties :D Just try it if you find some time. We play it on 2+ PC-s.
Victor Vran (2015)
Just bought it some days ago, but so far its good :D Gerald from Witcher 3 is giving his Voice for this game.
It's a nice Beat'em'up game :D Not the best one, but not bad if you need a new game :D
Kopanito All-Stars Soccer (2016)
I played FIFA while elementary school and PES in the High school. I thought I'll never play soccer games again, because i started to dislike the sport, but this game is awesome :D No fouls, awesome graphics, nice game play and a lot of fun :D 5 people can play it at the same time :D
Left 4 Dead 2 (2009)
It's a cool old school Zombie Shooter. We play it on 2 PC-s.
Torchlight II (2012)
One of the most awesome Diablo style games. You can add Custom Mods to have over 50 classes and adds and keep the game still balanced. Highly recommend it to everyone. We play it on 2 PC-s.
Mario Kart Wii (2008)
Dolphin Emulator + 2 Game Pads and Have Fun :D