I just recently finished playing Halo:ODST and Halo: Reach. The co-op in both games, was just as fun as the first 3 Halo games! I liked ODST a little more because it had more of a fun, lighthearted story. Reach’s story was a bit too dark, concerning the characters, but the gameplay was on point as always.
I still have my original Xbox and Xbox 360 along with Halo 3. Never bought an Xbox One…I also need to buy back the first Halo, and Halo 2 eventually…
Haven’t played a lot of co-op but those were always a blast whenever playing with friends.
If anyone’s interested in Halo 2 on PC, Project Cartographer brings online support back. If we’re talking Halo 1, the original PC release and Custom Edition still work fine online, even now. Custom Edition is actually a favorite with a few friends of mine, a lot of amazing custom maps and mods on that one. I’m not sure anyone’s figured out any way to do co-op unfortunately in either game.
It’d be great if a couple of us played a game or two though. 
So does everyone have a PC capable of running Halo 1 or 2? I imagine even laptops integrated GPUs would be able to handle it. And if you have a Mac you can install Windows 10 via Bootcamp. Installing the game itself might be the hardest part, given that many (most?) laptops don’t come with an optical drive anymore and it don’t recall Halo 2 being re-released digitally. It might be better to choose another game from around that era that is still supported instead.
If we’re talking about Halo 1, Custom Edition is a standalone multiplayer download that comes with all the stock maps but with the extended capability to play user made ones. All you need is your retail CD key during the install. It actually has many faithful recreations of Halo 2 and 3 maps and they work fairly well within the limitations of the Halo 1 engine. Ancient potatoes could probably run either game at this point and if you can get them installed, you should be good to go.
Well the one game I play these days is unreal tournament 1999 , it still has a lot o people on it’s servers and kick-ass gameplay and you can get on steam.