I've also mostly enjoyed online multiplayer games, but I play a bit of everything. I think these are the games I've played most in order it must be:

World of WarCraft - Been at it since a year or so after it originally launched. Played a lot of Guild Wars 1 before that, and also lots of other MMO games before and in between, but WoW is the one I always keep coming back to. Longest break must've been like 6-7 years around the Cataclysm expansion, and then back at it at the end of Legion. It's on the back burner now again because of another mediocre expansion, but I still check it out occasionally. I think I must've sank at least 5000+ hours (probably way more) into this game over the past 20 years.

Elite Dangerous - Been backing it since Kickstarter, had high hoped for the game and it was fun while it lasted. But I lost all hope in Frontier in managing this game. They're only focusing heavily on microtransactions, currencies and paid early access content now. Must've been about 1500-2500 hours or something, but I've been out of this one for a couple of years now. I had high hopes for the future of this game, but Frontier is a master of promising glory and delivering disappointment.

Squad - Last, but certainly not least for me, around 1400 hours playtime (including the testing branch client). Got into this game right when it launched into early access on Steam. Was very tired of the themepark rides that Battlefield and CoD were turning into, but didn't want to commit to Arma's milsim style either. Squad fits in between perfectly. Also one of the greatest game communities I've got the pleasure to be playing with, never had so much fun with completely random strangers. The mandatory voice-chat really ties it together to create amazing and fun moments. This game really taught me that voice chat in games can actually be good, and not just kids spitting insults.

That said, I have a lot of games on multiple platforms, that I usually won't ever hit a 1000+ hour mark on most of them anyway. Like I played Witcher 3 twice (and a half I think). But that's like only 150 hours or so total. And this year's favorite has been Helldivers 2 by far, but that's only been almost 300 hours so far.

Civ 6, Fallout 4

Team Fortress 2

Monster Hunter: Not all at once of course, but it's a game I keep coming back to. A friend in college introduced me to it on the psp and I was invested. I've had some version of it somewhere since, and I don't think I've ever removed it from my psp either.

Overwatch: This is from before the change, but it became a comfort game. There were no chats, and I didn't have to be in voice chat. I was low level (bronze), but I was content. I still play here and there, but I may get a match or two in before I leave out. Back in the day I could play for hours.

I have around 1700 hs in Destiny 2, and close to the same in Bloodborne. Over 1000 hours in the last 2 monster hunter titles. I've replayed resident evil 4 and castlevania sotn dozens of times.

So those are my most played games

World of Warcraft

Grim Dawn (currently playing Community Season 7! check it out @ https://grimdawnleague.com/, Dead Cells and Terraria (and most of that time unmodded - i just love building cozy or not so cozy homes).

In descending order: Skyrim Fallout 4 Starfield

Morrowind is a game that has extreme hours too, but not sure if it is 1000+ (yet).

Dayz.

Just hit 3.5k hours and it's still the most exciting game I've ever played. Nothing else gives the same adrenaline kick as a good encounter or a fight in Dayz. Also a great game for meeting new people as talking to others in this game is a must.

Would be arma 3, dayz and stalker series. My currently favourite is stalker Anomaly.

Monster Hunter World and Don't Starve

Empirically, Final Fantasy 14. Also my only 1000 hour game since games services started logging playtimes in a more durable way. Only other games I can think that might have touched that time are Diablo II and UT99, but both of those playtimes are lost to the sands of time.

Satisfactory

Monster Hunter

The only two games I have that much time in are Factorio and Satisfactory.

Wow, so many posts 😵‍💫 Sooo I think I'll be the one to mention Elder Scrolls Online ^.^ I've got 1800 hours in that and miss it pretty bad v.v Am stuck on a laptop that can't run it (nor much else) playably :-\ Anyway, it's got many things to do. Three faction storylines, lots of side stuff (everything can be played at any level; people will take low-level characters into Trials and not even worry about it), crafting, character build funs deeper and more interesting than they seem, just plain wandering around peekifying all' the Elder Scrolls places... Idunno, it just somehow kept me of all critters hooked for quite a while, which is very odd. I tend to get bored of things realquick 😅

I only have 800 hours in it but I suppose I can second (or, well, nth) Warframe? 😅

Elder Scrolls Online, as long as you find a good group of friends.

I also nearly have 1000 hours in MGSV Phantom Pain.

Path of Exile. Hands down.

I just broke 1400 hours and still going strong. There's so much to do and so much to learn, and it's so good at rewarding grinding and keeping you chasing those incremental improvements. It's 100% replaced RuneScape for me.

I have broken 1000 hours with Cookie Clicker, Guild Wars 2, RuneScape, and Eve Online. I don't recommend the latter two anymore, but CC and GW2 still hold up.

Honorable mention to Factorio. I'm still in the hundreds but it's climbing.

Stellaris. I'm almost at 1400 hours in the game and while I now play it a lot less often than in the first 1000 hours, I still get the itch to play it again a few times a year.

Don't Starve Together scratches the MMO itch for me. It's not an MMO, but there are public servers where you can hop in and hang out, raid bosses and whatnot. I have ~4k hours in it now.

Minecraft...

For some context I have insomnia. Stellaris isa game I've been able to fall asleep playing. Eventually it auto pauses but that's fine. It's a game I will always play because I find it peaceful and relaxing

Apart from Flight Simulator 2020 and DCS, I absolutely love games like Euro Truk Simulator 2 and Snowrunner. I put ungodly amounts of hours into those. Especially ETS 2 is incredibly relaxing. No pressure, just a lovely drive. It’s definitely not for everyone though.

Alan Wake 1. It's one of the few games i can play over and over again and not get bored. Sometimes i'll just pop in and play a chapter when i want something to do. I keep it and it's sister game, Alan Wake's American Nightmare on system at all times just so i can play it

DOTA 2 is my primary time waster, have over 3000+ hours on it. It's endlessly entertaining, because there is so much variability in the games. You have 4-8 spells depending on the hero, with 6 active item slots, which means once your team has leveled up, team fights can be a burst of seconds, or a 5+ minute long affair, with each tank employing different survival strategies to block damage, heal, or escape, just to heal and get right back in it. There are multiple different game modes, but are a little challenging to find and play. Still, it is the one game I play with the steepest learning curve. I don't think I saw myself as "good at the game" until I was around 1000 hours in 😶‍

Smite is the only other game I have 1000+ hours in. I started playing it as a break from DOTA 2, and as a way to connect with my little brother. It's a lot like DOTA 2, but far more simplistic. Each god has 4 spells, with no active items. Instead of left-clicking for movement on a map, you use WASD to maneuver your god. It's fun, and you can pick it up after like 30 hours. The different game modes are a lot easier to navigate than in DOTA, but it's also a lot less engaging. The updates and quests are all very simplistic. I was going to try out the new Smite 2, but after watching some game play, it looks... exactly the same as Smite 1.