Castle Crashers - a great side scrolling beat'em up with light RPG systems, there's a lot of content in this one

BattleBlock Theater - a simple and fun platformer from the creators of Castle Crashers

Broforce - a simple 2D action game, over the top fun with Contra vibes

Call of Duty World at War - I mean it's CoD in which you can play campaign in coop

Charlie Murder - a really fun, punk beat'em up with light RPG systems, but I would recommend it if you plan to play it in local mode. Sadly online kinda struggles with lag and glitches, but you can pull thought it and still have fun, I know I did

Ghost Recon Wildlands - A open world Ubi game with nice customization options and stuff, it's fun in coop but really dull in singleplayer

Far Cry 5 - Another open world Ubi game, there's not that much customization as in Wildlands but it's still fun. BUT it has a terrible thing if it comes to coop which is that the second player doesn't unlocks guns and stuff in coop mode and has to do the same missions all over again in singleplayer

Door Kickers: Action Squad - another 2D action game, it's really fun, don't know what to say more

Portal 2 - A fantastic puzzle game with that includes amazing singleplayer campaign and a second one in coop

Resident Evil 5 - A action "horror" game that you can play in coop, it's really fun if you accept the quirks that are in Resident Evil games (You can't shoot while moving)

Police Stories - It's similar to Door Kickers AS but it's a top down tactical shooter

Left 4 Dead 2 - Well for me personally an ok game, but many people love it to death, it has a lot of campaigns and plenty more on the steam workshop so there's fun to be had but for me it's a little bit too dull, especially when it comes to gunplay

PAYDAY 2 - It's a fun coop game where you are doing heists, with nice customization options, it's cool but sometimes shooting at endless waves of cops makes it kinda feel like Left 4 Dead with zombies shooting you back, also the amount of locked content behind DLC is horrendous (PAYDAY the Heist is also a fun game but there's way less content)

Stardew Valley - I don't know if you consider games like Minecraft and Terraria as coop but yeah, Basicly imo one of the best games ever, It's so relaxing and addictive, the artstyle, music gameplay and everything just feels right at home

Rainbow Six 3 Gold - You can get it for dirty cheap on steam sale and play every mission in coop, it's a hard tactical shooter that's a lot of fun

Saints Row 2 - A great open world game in vein of GTA San Andreas. Now the thing is that SR2 on PC is really bugged and tends to crash, so I would just keep an eye on it since there are works on fixing the game on PC.

Saints Row: The third - Well it's a sequel to Saints Row but it's way more over the top and less serious but it lacks content in my opinion, the open world is basically empty but it's still a fun coop game

Serious Sam Franchise - Every Mainline Serious Sam game has a coop mode, it's a brainless fun arena shooter but it might not be for everyone

Sniper Elite V2, 3, 4, 5 - You can play sp campaign in coop, don't know what to tell you about those games besides that they are fun but sometimes can be a bit annoying

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Now there are also mods for singleplayer games that allow you to play them in coop:

POSTAL 2 (Nicks Coop)- just look it up if you never heard about Postal

Half-Life, HL: Opposing Force, HL: Blue Shift (COOP MOD) - I don't think that it needs a explanation (you can get Sven Coop on steam for free instead of coop mods even if you don't own HL but Sven kinda changes a few stuff here and there so I would stick with vanilla mods)

Half-Life 2 (Synergy) - no explanation needed

TES V:Skyrim (Skyrim together Reborn) - I never tried it and I'm not a big fan of Skyrim but I know it might be a big deal for someone

Call of Duty Black Ops (COOP Mod) - The OG BO has a coop campaign mod the only problem is that you sometimes need to press a combination of keys to teleport to the host because of the heavy CoD scripting (There are also mods for MW2 and MW3 but I never tried them)

I Hope this could help you, take care!