Borderlands Series
Borderlands is the original loot-shooter. It can be played alone but having up to three more friends drop into your game to find new weapons together is when the series is at its best.
The older games have recently been re-released, so you might be able to find other people to play with, but it’s probably best to wait a couple of months for Borderlands 3 to release.
With more missions to tackle, a new world to explore, and an incredible number of weapons to find, it’ll be a big step forward for the series, without losing the lighthearted side that made co-op in the original games so fun.
Whether you want to grind with friends to level up quickly, or just mess around in the game’s varied world, the Borderlands games are the pinnacle of co-op first-person shooters.