Divinity Original Sin 2 sounds like it would be great coop. Get a friend or a few together and have a blast in this game.

Shadow Warrior Classic, it´s free, funny, gore and works well on every computer.

Dark Legions is a great co-op game.

Easy
One friend? Divinity Original Sin 1. Three friends? Divinity Original Sin 2

I think the best games for co-op play are ...... Nintendo. Sorry!!
Yes, no PC games but what a pleasure to play Mario Kart with four or six people, smash bros or games like Wii Sport.
What a joy it is to be with family or friends, grumbling, shouting and laughing in a good atmosphere.
A great way to have a great time.

Sonic 3 & Knuckles. That's a co-op classic!

Knights and Bikes. Lovely bit of escapism for twp players. Got me and my g/f through lockdown #1

This might be a nostalgic one, but I'll have to go with Freelancer.

Sitting side by side, scouring nebulas for shipwrecks, battling it out with whomever you chose to oppose; getting those bounty hunters off each other's back; finding lucrative merchandise to trade; sharing the joy of having discovered something new: a hidden base, a beautiful vista, a huge battlefield riddled with shipwrecks somewhere in the outer systems, or even a meagre jump hole.

Getting from one point to another was quite slow – adding to the feeling of vastness –, and required only some interaction, given that you weren't attacked, which made it possible to admire the views, get a slight feeling of insignificance, and talk to your fellow freelancers¹. The further you strayed from the core systems, the less trade lanes there were, which meant even slower travel. In those moments – especially if you hadn't played it before, or at least not in a long time – there'd be a real sense of piercing the unknown, and first and foremost: togetherness.

There was a real feeling of discovery and remoteness: you were adventurers, pioneers, freelancers, seeking your fortune on the edge of humanity's realm. Discovering huge bases beyond the rim of civilization, finally finding whoever sold that ship you searched for for hours on end, only to realize that you didn't have nearly enough credits to buy it; those where the things you played for. And you were in this together; you'd feel closer to the people you'd play with. And that is what makes a good co-op game: Making you feel less alone in an unforgiving, uncaring cosmos, no matter if you're traversing it in a virtual spaceship or sitting in front of a computer only pretending to.

Granted, many co-op games achieve that, but for me there never was anything quite like this before or after.

We still fire it up every couple of years.

¹ Side note: my friend's laptop's cpu fan was shot, or at least didn't work properly, making it necessary to almost constantly provide a cool breeze with a book, so the downtime was desperately appreciated.

Can't go wrong with Phasmophobia! Spooky but fun game to play with 2-4 people, just got a huge update that changed a lot of mechanics for the better.

For me. Men of war

Meadow, Risk of rain 1 and 2, Titan quest 2, Muck, Out of space, Diablo 1 and 2, most of the lego games, No man's sky and Retro City Rampage.

Shock Troopers one of the best NeoGeo arcade shooters. Non-stop action, explotions, a great roster of playable characters every one with pros/cons and alternative paths. So to make it shine to it max grab a friend and play untill both become unstoppable killing machines.

From games available on GOG I can easily recommend first Serious Sam for adrenaline and fun with friends, but not sure about latest game since haven't played it.
Among other games I can recommend two games I have discovered over the last year. The first is A Way Out. If you like good buddy movies, this is the game for you. Interesting main characters, good voice acting, many memorable moments - all this is present. It's not very long and can be completed in one run, but personally gave me a lot of emotions. Also haven't played the next cooperative game from this developer, It Takes Two, but looks like it received even more acclaim.
And the second is charming puzzle platformer Unravel Two. I really liked the idea of tying the two characters together as a symbol of the bond between friends. It's also not very long if you don't get stuck on puzzles, and also have additional hard challenges. The only drawback is the lack of online co-op. Although i think the original Unravel was a bit better, perhaps because it triggered a stronger emotional response in me.
Oh and one more mandatory recommendation is Portal 2 of course, a unique experience.

Doom as in the 90s. One player moves, while the other shoots.