According to Steam, it's Skyrim at 788 hours, followed by Crusader Kings II at 615 hours, followed by Skyrim at 522 hours.

Honestly though it's probably Pokémon FireRed, my first ever save was over 500 hours when I eventually reset it, and that's not counting all the hours on an emulator

Star Wars: Knights of the Old Republic 1 and 2.

I replay them both multiple times a year and make sure I get as close to 100% each time.

I wish The Old Republic would've failed as a subscription because I whole heartedly believe it's the sole reason we'll never get a KotOR 3.

Definitely The Binding of Isaac Rebirth. ~800 hours.

Either Minecraft or Animal Crossing. Both are far beyond 2000 hours, but I don't know which is ahead.

My answer will likely always be World of Warcraft. I played in my 20s and I played a lot. At some point when I typed /played, the number I got back exceed 1 year of in game time. I vowed never to look again out of shame. I this day I do not know what my total play time is.

I’m not proud of it but most likely League of Legends. It was pretty much the only game I played along with csgo in 2016 when I was a teenager. Dark times. I don’t know how many hours I have in League but in csgo I have 216h which compared to some people here isn’t that much lol.

EVE Online and Valheim.

Endless Space 2. 488 hours. It's my comfort game. I wait long enough to forget how to play it, then I play it again.

I'd have to check, and for the older ones I can't, but either something from Bethesda before they forgot how to make games (probably Morrowind, possibly Skyrim or New Vegas) or something from Paradox (either Crusader Kings 2 and 3 or Stellaris), followed by Cyberpunk 2077 (can't keep me away from Night City for long) or something else by either of the first ones. Or Excel; wasted a few thousand hours on that one too.

Diablo 2. No idea how many hours, easily over 3k for sure.

No other game I played come close to this amount, closest is probably 600 or 700 hours. I got so many 50-400 hours games.

While not in 1000+ hours like others, but I have 700± hours in both Terraria and factorio.

The Binding Of Isaac, around 1700 or something

I'm sure when I last checked GTA5 on PS4 was sitting at around 1500 hours.

MW2 back on 360 was sitting at around 35 days according to the multiplayer leaderboards.

This old Korean MMO called Nexus TK. I think it was Nexon's first game (Arc Raiders). Picked it up in like... 1997? I played it very heavily for a few years on and off. When I say heavily I mean like 4-5 hours a day. It got bad at some points. Got off of it at times but always went back. I still pop on every now and then just to say hi to the hundred or so people who still play.

Civilization V, especially considering it might often stay open all weekend on the second monitor. Also saw Eve Online mentioned; that took untold amounts of my life, but I've been clean for a long time o/. Arma 3 with it's various mods also has well over a thousand, what with it's endless modular gameplay and annoying ass realism (hiking sim). But right now at this very moment I've got X4 going on a few hundie. I've heard it called Eve Offline; I dunno if it's exactly that, but it can be incredibly frustrating and clunky, but also crack cocaine. I think I've just barely figured out how to play it.

Depends how we're counting. Actively playing is probably WoW, but I have so many hours in Factorio, as i would just leave it running overnight. Had a server up for friends also, so probably have close to 1k hours of the game being open, but definitely feel like i wasn't actively playing as much as WoW. Just to how many years i put into playing WoW.

I have 5133 hours in Transformice. It was pretty much the perfect game for me at one time.

It may look like a silly nothing game, but the skill ceiling is almost non-existent. I felt like I was improving for every single hour I put in. Made a lot of international friends, too.

Probably Path of Exile 1, but fortunately I got it direct from GGGs website so it didnt track hours.

Try out POE 2 when it goes free to play!

Guild Wars 2 with 9405 hours as of just now. Non-predatory MMO with fun moment-to-moment gameplay

Put a lot of hours into Paradox Interactive games

Elite Dangerous. Instead of paying attention to my uni classes I was hauling freight to Robigo and slamming people with a hull-maxxed federal dropship.

I don't have that much free time to play as an adult anymore and steam is only tracking game time from adulthood.

Mostly a mix of ARGPs and industry games, because the mechanic is engaging, simple and repetitive. I generally avoid story based games as by the time i can play again, I've forgotten what the details of the story we're about. I need the "in last episode" for story based games.

Top3.

POE 891h. It's a really good ARPG game. Satisfactory 538h, industry part. WH 40k Inquisitor(combined) 532h, Warhammer themed ARPG.

Battlefield on PlayStation, hands down

Played 2, 3, 4, 1 (don't ask, I didn't make the numbering) and 5 to death on a wide variety of play Station models. Would log some average of 10 hours per week, every week, for since BF2 started and BF5 ended

I don't have a log of hours

I don't play it on Steam

but I'd be pretty confident that it'll be Cataclysm: Dark Days Ahead.

https://en.wikipedia.org/wiki/Cataclysm:_Dark_Days_Ahead

Excluding MMOs, Captain of Industry. Otherwise WoW, of course.