Alright first of all, since Zandronum has become one of
those open source projects, you're going to want to have the latest beta/alpha/whatever instead of the latest stable which is woefully out of date. Should be easy to find on the Zandronum page download page.
https://zandronum.com/downloadI don't even remember where Delta Invasion is anymore, nor where the skulltag.pk3 compatability thing is for actually running it. I hope SK (or even Soap?!?!) remembers more than I do on that front.
Co-op is pretty radcore though and there's always great stuff because you just put in a regular ass doom WAD and out pops fun! Some of my picks (I'm going for more rather than less here):
Ancient Aliens is the latest magnum opus of the Doom world, and it's fucking amazing. It's fucking pretty too.
Plutonia The IWAD: Still good after 20 years
Community Chest
2,
3, and
FOUUUUUUUUUUUUUR: Just a good selection of maps by people trying to make their big splash with their potential only shot at mapping. I forget which one had Soap's maps (probably 2) and which one had Literately The Entirety Of Death Mountain From LTTP The Map (probably 3). Two might be rougher than the others in varying quality land.
The original CChest is quite a bit more hit or miss.
Memento Mori: There's a lot of old classic we could talk about here but MM, though it's one of the oldest, does stand out in that it was actually built primarily for co-op play, so you know. Pretty good, with only a small amount of "for its time" rider.
Scythe: Another old classic that I point out mainly because it's filled with shorter, smaller maps on purpose, so if you want that kind of pace, there you go.
Scythe 2 doesn't fill the same niche but I should point out that it's damn good too.
I'm not sure many of you are interested in DM but I'd be down and
this seems like the mapset of choice for now.
ALSO SOME MODS IN CASESPACE HUNTER: It's an NES Metroid weapons mod for Doom! May not sound exciting, huh? Well it's pretty great; I'll convince you by mentioning that the ice beam causes enemies to explode into ice shrapnel, which can kill other enemies that also explode into ice shrapnel, which can... you get the idea. There's some dumb shit in here.
Samsara: So I have no idea where skmega is, whether it still works, and whether SK can bear looking at the vault of time capsule meme horrors within his soul, but apparently he wasn't the only one with that basic idea. The selection of heroes is quite a bit different from SKMega too, with only a bit of overlap.
Samsara