Id say Doom.
Actual Doom or the BoS that came out in 2016?
Europa Universalis 4 Multiplayer is really fun
Don't lie ! It's a disgusting poorly designed boring ass RTS game, RTS games aren't even all that bad, but EU4 is not meant to be played as such.
I highly recommend Modern Warfare 2, it's a short but good singleplayer game and the co-op is very challenging, but never fully out of reach. Frustrating, but in the best way possible. It took me years to finish all special operations co-op missions. Modern Warfare 3 is likely fun, too. Mostly forgotten games in the Call of Duty waves, and perhaps rightfully so, contrasted to Modern Warfare 1, but MW1 doesn't have Special Operations, just a short campaign with a good plot (if you're into that).
I almost beat the whole campaign on Veteran on Mac. Using mousepad. It was harrowing.
Anyhow, the missions in Modern Warfare 2 are varied, and they're of course somewhat linear, but it's hard to balance non-linear strategies, and they're never the same due to loads of randomisation of elements, mainly where foes spawn, but the randomisastion itself is never that noticeable and luck-based as it is like in Payday 2 (I got it this free day, would not recommend, it gets boring after 20 hours).
Playing Doom (not Doom 4 piece of shit) and it's wads is mandatory. It's damn good, and LONG, even if you only play on official maps, made by folkd paid by id softtware. And then there's a BAJILLION community made modifications and level packs. My favorite is "Hell Revealed". I'll just say, it lives up to it's name.
Halo 1 (and likely 2) are damn good games as well. They're tough as nails on legendary. For example, the first 2 weapons you get is a pistol and a submachine gun (it's technically an assault rifle but it's super innacurate and the developpers even implifed after the release of Halo 2 that it's more like a SMG rather than an AR). There aren't that many weapons (10? and you can only carry 2 at a time), there aren't that many foes (12?), but they're designed well, and the maps keep things shaky as well. Anyhow, with the first 2 weapons you get, on Legendary hardness, it takes roughly the full clips of either gun (pistol or SMG) to drain the regenerating shields of alien squad leaders (elites). Each encounter with a squad is very tough, it doesn't get old. Now, you can use the sniper rifle, grenades, or a rocket launcher, which kills any elite in one hit but those you got to save for even worse settings.
The multiplayer is fun but they took down the servers so you have to get something unofficial working and I never got it to work.
Edited 6/15/2017 20:24:49