So going to go all lawyer and point out that it reads "coop game" and not coop video game.

So I'm going to go with some classic D&D tabletop. Always a great time to be with friends and having a great soundtrack in the background.

Worms World Party
Played a ton of matches versus my cousin back in the days when we were young. And the two modes we always used were no weapons, crates only & crate shower. It lead to interesting games and epic turn overs -> falling badly behind and got Armageddon in some random crate -> instant kill everything XD

That was what we played when we were taking break from HoMM3 hot seat, ofc... 8-)
Best game ever, we even memorized most of the creature stats, and music was epic, especially Rampart town theme (my favorite faction). Will always remember moment when my friend resigned in first week of the game when I got Tome of air magic from a shipwreck survivior... 8-D

I just hope if I win something it won't be third prize. I got all those games, my first purchase on gog and the reason I joined in the first place, to have all my heroes with me... 8-P

Stay strong guys...

Neverwinter Nights

My all-time favorite co-op. With community-made modules it is a near endless bounty of gameplay.

System Shock 2, obviously, because none of your team members had it on their recommendations page, which is heresy of the highest order!

Unsighted of course!

Easy thing: It Takes Two :)

Get Medieval created by Monolith Productions for Windows in 1998.
It is top down view action fantasy game in which the player can control one of four characters in a dungeon.
It can be played in three modes (Dragon Quest, Random Dungeon, Custom Dungeon) and on four difficulty levels.
Both hot-seat and network multiplayer are supported. Happy Anniversary GOG !

There can be only one choice for any party

Unreal Tournament!!!!!!!!!!!!!!

GODLIKE......

Its safest to recommend a shooter. IMO Rainbow 6 Siege is the best one out there. The destructability, tactics, map knowledge, operator diversity and optimization make all other shooters boring for me.
How many shooter out there are where you can shoot a single bullet and camp people through the hole...

All you have to do is to recommend a coop game to play during our Anniversary Party:

And recomend i shall ! There can be only one game ! : "Jones on the fast lane"

Heroes of Annihilated Empires (https://www.gog.com/game/heroes_of_annihilated_empires)

It is one of the best rts games around. The graphics still remain awesome to this day. The battles are Immense, the Campain was Amazing, with it Showing Technology of airplanes, tanks, jeeps, that had managed to find a way into their world to invade. If this game had continued, we would have had an Expansion pack to rival most games now.

You can't go wrong with LEGO Star Wars. The Force is strong with this one.

Gauntlet, Cabinet 4 man. Mid 1980's

4 friends crushing mobs in a Mall Arcade.

Best Co-op +Video game, you have to go very old school.
Back when video games cost only a quarter per play.
When MMORPG was only 1 letter, M for multiplayer.
To be alive during the beginning of the video game era was the shit.
Remembering having a pocket full of quarters and rushing to the change machine for more and hoping the machine wouldnt spit out your crinkled dollar.

Yea Im old and proud of it.

If your doing local coop, you should do Enter the Gungeon. The guns and gadgets are delightful with all sorts of unusual offerings. There's a gun that's a bullet that fires guns that fire bullets, a gun that fires tears, a gun that is riding a skateboard which increases your coolness, and most importantly if you start as the hunter you can pet the dog. It's one of the few games I've gotten 100% and kept playing.

I know this is going to sound ass kissing, but this is what I used to do with dad and we had a blast doing it: HoMM3 hot seat.

If you want answers that don't involve Heroes of Might and Magic:

Teenage Mutant Ninja Turles: Turtles in Time or Diablo 2 if you like your friends.
Mario Party or Mario Kart if you don't like your friends.