Have you guys heard of Unholy Bosses? It's basically Versus Saxton Hale except there's multiple classes and you spawn with items that essentially can change the round, for example the rubber band does 10k damage, there are also superbosses that are way more powerful thatn regular bosses, there also special rounds that consist of you fighting against others as a "Demoneye" or "Devileye". It's quite fun and it kinda explains itself once you play it.

Among the things I remember, there's been mods based on Megaman Soccer, Battle N' Chase. Also a zombie mod named Roboenza - that was way before, it thrived towards the 8BDM v2 era.
There even were mods that translated Smash Bros and Ghouls VS Humans into MM8BDM.
I guess Roll's Chaos Generator is a classic - player stats changing, and crazy things spawning in the map.

i dearly miss justiderp . its been so long (a decade now lol) since i was last able to fiddle around with that mod but i remember having such a good time playing with my friends on the blue bomber bed & breakfast map with it . sometimes i still hear tf2 heavy uranus in my dreams ...

Off the top of my head i can think of two mods
-GvH arcade should be played more often, i want to induce paranoia on people with Boo or schmove as Cyborg among other things, part of me prefers GvH in 8bdm than in doom mostly because it stays pretty close to vanilla GvH rather than the GvHaddons made on doom that went a little crazy (to say the least) and are played more than vanilla GvH, i like the condensed small roster than the expansions and i'd say it is still fun enough with the few guys that are playable. I'd love to have the GvH mappack from doom ported into mm8bdm in general but i'm certain there's more than enough reasons why that's not happening. But its compensated enough by mm8bdm's vast amount of maps that i dare to say can compete with GvH's best maps and its enough variance that makes it enjoyable. Also the 8-bit jumpscares are very funny.

-Unholy Bosses is still a hilarious mod to me, i can't call it underrated because the mod's not exactly abandoned cause it's still in development and i cannot call it forgotten because it still gets active servers, but i can call its game design underrated because for how crazy the entire mod is by having friendly fire, buttload of BIG damage and a LOT of randomness results in a mod that has a surprisingly good replay value as not all rounds flow the same and even if you get a lil toxic from back and forth teamkilling, at least it gives you the chance to fight back and even the score and fighting bosses in different patterns (e.g. bosses meant to be fought in a specific team appear alone and mixed with other 1-3 bosses). Game's just mario party with guns and i'm all up for it.

i'd say LMSGames, it was such a fun and chaotic experience back there with the random stuff that we had to do to win one, specially with some specific minigames that required cooperation

06Saga is pretty fun and even had lore

8-Bit Death Smash was a fun (although kind of jank) time. I haven't seen it been hosted in a while.

Nobody's talking about Spring Cleaning (edit: oh wait never mind dood mentioned it lets gooo), an attempt at recreating Prop Hunt in 8BDM, but it was incredibly fun when it happened, and honestly something no one in the community really thought to try at the time. And its understandable that no one's talking about it, given the context surrounding its creator, I wish someone would recreate the mod concept tho it was fun

Someone already said it but Roboenza was really fun, was basically infection from Halo, I think in the original version the zombies were slow and super tanky and they changed it to make them faster with less HP, they could also climb up walls. I still remember the sound effect and text pop up when you get infected, great time, hopefully someone has the wad for that lying around somewhere.