I'm looking for the Holy Grail of multiplayer gaming

Unreal Tournament 99.

I've spent countless hours in the 2004 version
To this flavor: splitgate
I was going for counter strike classic, but yup.

My interpretation of your request boils down to "what's a good co-op roguelike" where the grinding is the replaying.

So, depending on how many players you need it to support and preferred genres, you might check out games like

Risk of Rain 2
Enter the Gungeon
Children of Morta
Vampire Survivors
Streets of Rogue
Gunfire Reborn
Barony

There's also a game called Jumpship that i'm keeping an eye on the development of that's supposed to be hitting early access in the coming months.

Sea of Thieves.

There has never and will never be any pay to win. Everyone has access to all weapons at any time, no unlocking, just pure skill.

Up to 4 players on your team (ship) open servers with other players all sailing around all the time. You can get in an organic fight over treasure, or matchmake for ranked battles.

All of the progression is cosmetic based.

The devs have been adding content constantly since launch that fleshes out the game systems and makes for more interesting interaction.

I come back to this game all the time. Highly recommend.

This game is so much fun, even when I’m the loner getting my ass handed to me on my sloop (which is most of the time). Seeing a ship looming in the distance and wondering if it’s going to come after you is such a rush.
Is it worth getting this late?
- Absolutely, it's still constantly getting updates, and got an actual anti-cheat recently which helped the health of the game a TON.
  
  Huge learning curve to the game, but there's nothing else like it.
- Definitely, even better if you can try during a free weekend or get it half price on a sale.
  
  Amazing with friends, on your own or with randos.
  
  So many organic ways to interact and communicate with other players. Seriously cool, unique and worthwhile experience.

When you ask for something without 'grind' I have to ask if you know what you are asking. Grind is entirely subjective. It's not a mechanism of a game but rather what happens when you personally don't find a game mechanism fun/rewarding.
Take classic examples, like mining in... most games, really. It's smacking a rock. It doesn't have much variety. For some people, they love their own little game of 'hit the rocks in the most efficient way,' or they like to relax with music and bust rocks, or they feel like every rock is a loot box. Other people hate it for being too complex to automate and too simple to feel engaged.

The difference between 'grind' and an 'endlessly replayable part of the game' is how the player looks at it. You are asking for 'the drug to which you will never build a tolerance.'

Chess.

- -
✍︎ arscyni.cc: modernity ∝ nature.

At the tightest-possible definition of multi-player
lichess good
Checks out

What do you want? Lots of games fit that description.

In some shooters, you basically only grind for cosmetics, like Overwatch, Marvel Rivals or Counter-Strike. Dota 2 is a Moba and also only has cosmetic unlocks, I think.

Maybe Minecraft or Terraria? I don't think you really grind in these games, but you do very similar stuff all the time.

I consider digging hours for diamond as a form of grinding, and you have to get it for crafting advanced stuff. I could play creative mode, but at that point it's more like a 3d Paint than a game
- Vintage story has grind also but it makes it way more interesting and satisfying.
These are the features that I want to avoid: grinding for resources, cosmetics, and anything that require repetitive and/or mindless action. Any genre is good, the question is more like a test to see if a game like this can exist
- No cosmetics pretty much disqualifies almost every game, because all of them have it, even old games.
  
  In my comment before, I probably wouldn't describe cosmetics as a grind. You just unlock stuff on the side, as a bonus, since most of it is random. It's not like you're going to play a thousand games of Ursa in Dota to unlock some rare skin.

Fighting games:

Endlessly replayable because there's always more to learn.
No grinding because there's nothing to grind.

Trackmania, although depending on how you want to slice it, you might consider it ONLY grinding.

Incredibly low skill floor (4 button racing sim) but with near infinite skill ceiling as you learn to master all the nuances of movement, surface types, tricks, etc.

Endless amounts of content with the seasonal campaigns, tracks of the day, and weekly shorts, but also just a full blown track editor for community content on the side. Each track is like a little puzzle where you memorize all the details then try and get your best performance. Play in an online server with your friends and just chat, listen to music, or watch a movie in the background. Find your favorite style and master it: tech, dirt, NASCAR, lol.

It's my favorite game to just turn my brain off and drive.

I play FF XIV online. There's as much or as little grinding as you want there to be. I'm in a really chill social FC.

Can you advance in the game without any grinding?
- Yes, they give you the gear you need to make it each step of the way through the story.
  
  Crafting and gathering are optional. Relic weapons are optional.
  
  There are some players who like to role play. There are casuals, hardcore, and everything in between.

Deep Rock Galactic!

I've played it, but it has become a bit of a grind. The only reason for doing mission has become unlocking stuff
- Personally I've always just played the game to play it. I only ever cared about a few of the cosmetics and have long since unlocked everything just as a sort of byproduct. ...The vanilla gameloop can definitely get repetitive and stale, so I play a lot of modded games these days.
- I had issues with grind as well since the unlocks system required every single person to start at the same time and not play alone, as everyone had the same missions.
Works just fine with anywhere from 1-4 players

You can have a mix of friends and random players if you want a full team but don't have a group of four to play with

The game is strictly cooperative so there's no incentive to screw each other over, fostering a generally good multiplayer culture

There's lots of stuff to unlock but none of it is necessary for a fun and functional build

Teamwork is encouraged and effective

Missions are randomly generated but you've got years of additions to the random pools for lots of variety

Absolutely no pay to win stuff

There's cosmetic DLC, but there are still absolutely loads of great cosmetic options in the base game

Edit: also, rock and stone
- Works just fine with anywhere from 1-4 players
  
  Madding can get it up to 8 without issue and 24 with extreme lag and instability.
- For Rock and Stone!

Old School RuneScape.

Some people will say it is grindy, but those people are just weak minded.

People who imply rs has grind in it disgust me

The obvious answers are the games we endlessly replayed historically: Mario Kart, Goldeneye (VS mode), Halo (VS), Smash Bros.

If you specifically want ones on PC, I'd suggest Starcraft, Age of Empires, and probably Counter Strike (I wasn't into that one, but it had a huge following).

Many board games fit the bill as well. Codenames (physical or online at horsepaste.com) comes to mind, and another commenter also mentioned chess.

Basically any games that were made before endlessly grinding became a thing (yep, that's only been a thing for a decade or two).

Does Factorio count? It's a good game, you can play multiplayer, the factory can always grow (at least until your hardware, or in the extreme the software, can no longer handle it) and if you're grinding for something rather than automating it, you're doing it wrong.

I guess it counts because you are not grinding, the factories are grinding for you. The problem is that it's a niche game and not many of my friends would play it
- Its a wildly popular niche game for good reason.
  
  It's the defacto automation game, and can get pretty wildly funny with multiplayer co-op. Players slot into niches and ted to focus on building out X or Y and when these things meet can be hilarious.
  
  It also has a versus mode where you race to build bases on a shared map and kill your opponents first.

Tetris effect connected.

It made game of the year for a reason.

I think I've sunk 200 hours into tetris this year alone, I have it on like 4 systems including my SP handheld clone

Edit: my phrasing here was unclear, there effect connected has not been ported to portmaster or the ports collection, I use Tetris RR, a custom patched og GB tetris to have all the same amenities (hold, hard drop) as modern tetris.

If you like tetris and want tetris RR, find a GB tetris ROM and patch it yourself

A link to the Romhack (for the mods, this is a link to the patch, not the rom, i know VL has pirate content but this aint it, it does however have a link to their archive, where the rom does exist) https://vimm.net/romhacking/hacks/5813

Really depends on what you consider grinding.

Pretty much all MMOs or PVEs have you grinding for gear (helldivers 2 I don't feel is grindy in comparison, but some do)

Survival games like ark, valheim, etc.. Have you grinding for bases and the next section of the game

Pretty much all PvP games (CS2, valorant, apex, starcraft, Rocket league, etc...) have you grinding out muscle memory skills

The antithesis to these are instance-based games where at max you grind aesthetic gimmicks, but in single player games they don't have those like REPO where you always reset and fall guys where it is minigame based

The problem with these games is since you don't have a "reward for work" (grinding), people get bored of them.

Warthunder is really fun without much of a grind

I especially love how warthunder js a f2p game but doesn't feel like it forces you into parting with youe money.
- I agree :) and honestly paying for premium and planes you want is worth it. I have 750 hours and have spent maybe $100-200 on premium and planes. $3.75 per hour of birthday Monday seems decent to me. Better than my VR header which I’ve still only played down to $10 an hour
- Don't forget about the great bonuses for participating in the community forums, such as classified documents, toxicity, classified documents, and classified documents.

Beyond all reason

Fighting games. I've been grinding Skullgirls for over 10 years now, without a single skinner box in sight.

fall guys among us

Both infinitely replayable and has nothing to grind for except some cosmetics that doesn't affect game play

honestly check out archipelago, it's a framework that allows you to play a lot of different randomized games with your friends. you can play synchronously or asynchronously, and if you're handy with code, you can even add any game you want to it

appendix

"what's a randomizer?" a randomizer is a method of scrambling the items in a video game, while keeping it solvable, to be able to re-experience the same game with a fresh sense of progression. an easy game to think about this with is something like metroid or zelda. you need powerups to unlock certain parts of the game, but what if you could find those powerups anywhere you found a missile expansion or a chest? that's what a randomizer is

"how does that work with multiple people?" now imagine that, between you and your friend's randomized games, the items for both games could end up in either game. if we use the metroid/zelda idea from earlier, metroid might have zelda's boomerang, while zelda might have metroid's morph ball. the logic to ensure the games are solvable is still there, but you might be stuck waiting until your friend finds your key item. this is called "being in burger king" or 'being bk'd"

other vocab

check: any spot you can collect an item in a randomizer (think all collectibles and powerups in metroid, for example)

burger king: when you have run out of checks of your own and are waiting for someone else to send you a critical item you need to make any meaningful progress again. named after the first multiworld randomizer, where someone was stuck for so long, they were able to go to burger king for six hours and return only to still be in the same situation

Thanks, that's interesting

In all honesty, Rainbow Six: Siege is as ungrindy as any game could be, and it is as endlessly replayable as there are combinations of all the active players. The whole game is about finding ways to use the deep sandbox to outsmart your opponent, utilizing yours and your teammates abilities in unique combinations and it’s wonderful

Quake 3 Instagib.

There is a game i would recommend called Mechabellum. It's a mecha auto battler where you can't really move units between rounds. You build up a bug army trying to constantly counter your opponent with the 20ish available units.

SpiderHeck!

space station 14 (or space station 13 if you're nasty)

There is not even the early access

Titanfall 2

For me, it's Starcraft 2.

How's Bean Battles sound?