Have not played yet.

Re-release of Alien Swarm with now up to 8-player Co-op (4-player still available), more Campaigns, more Weapons, more Extra items and BOTs. The game is completely free, please give it a try!

L4D in the Alien universe. This game is only 3 player Co-op, but is well worth your time and money. It offers a Campaign and lots of progression and Weapons/Classes to level up.

Have not played yet.

L4D/Vermintide set in the Warhammer 40,000 universe. 4 players cooperate against zombies and other enemies in a grim environment. Ranged/Melee Weapons are satisfying and there's lots of progression.

Have not played yet.

Third Person Shooter but has a lot in common with L4D. This is the ultimate bundle to get the first 3 games. It gives a lot of the sniping goodness from the Sniper Elite franchise. Great Co-op fun!

Sequel to the first game. Play as the same 5 Characters, but you can now choose between 4 different Careers for each! It adds more Campaigns and Weapons to the first game! Highly replayable.

Left 4 Dead set in the Warhammer fantasy universe! Play as a Knight, Dwarf, Archer, Wizard or Witch Hunter! The rats have infested the city and you must get rid of this plague!

Deep Rock Galactic is a Co-op game where 4 Dwarves enter missions to find gold and other minerals. The game has a huge progression system and is very replayable due to its randomly generated maps.