Destiny 2. sigh. Before that it was Grim Dawn. Probably going back to that.

The most that I can prove is Europa Universalis 4, just north of 1100 hours on Steam. However, I strongly suspect my playtime in Guitar Hero 3 from high school likely exceeds that. Runescape might be another contender, but that was so long ago at this point that I don't really have a good idea of how much play time I actually put into it.

League of Legends, and nothing comes close to my time spent on it

The Outlast Trials.

Right around 400 hours

Most hours were probably EverQuest up to planes, counterstrike around 1.5-1.6, and TF2. I've had 200-300 hours in a few other games & series (elder scrolls series, dark souls series, final fantasies, etc) but those would all be well over 500. The first two were as much the community guild/clan as the game, the last was just a time in life where I was able to spend a lot of time on games.

Probably Civ 3. I've been playing off and on for over 20 years. I've played hundreds of hours of several other games, but civ 3 has to be the peak

Bloons 6 :)

its an unofficial number but rock band 3 i used to play every day, 5-8 hours a day. for around 3 years

Most likely League of Legends.

Distant 2nd is Path of Exile with ~5k hours.

Minecraft is probably also in that ballpark.

Terraria with 1k hours.

Stardew valley with 700 hours.

I also played a ton of Maplestory back in the day but have no idea or way to estimate how much.

I have spent around 2000 hours in Gmod. A sizable percentage of those hours involved trolling the people who play TTT. Escapades involve kicking off a detective civil war, provoking people into offing half the server, and spamming admin messages and false reports. The gamemode has fallen out of favor these days, so I only really check out interesting Gmod maps and nextbots now.

Civilization VI with the right mods is insane. Just a shame the multiplayer is so ridden with desync, and the newer ones suck.

It's either Genshin Impact or Animal Crossing New Horizons (it's Genshin Impact by like 1000h more lol)

Probably Ultima Online if I had a way of checking.

I used to play that game all day, every day I wasn't in school from 1997 to about 2004. I even still play it on occasion, just not on EA's servers.

Based on what Steam has recorded, ARMA 3. But I know for a fact that many hundreds of hours in that was just the game running in the background on the map editor while I typed up scripts in notepad++. TF2 is the next highest one, and that game was like UO again for me from 2007 to 2014.

I seem to get obsessed for 7 years and then just drop things.

Official? 4000 hours in Left 4 Dead 2.

Unofficial? Easily over 10,000 hours in Sven-Coop (a multiplayer Half-Life 1 mod).

I started playing Sven-Coop when it was released in 1999. I play at least a little almost every day I'm not out of town. I moderate some servers. I show new players how to beat hard levels. People still make new levels. Sven-Coop wasn't added to Steam until 2016, so the hours weren't tracked until then. Since 2016 I've racked up around 4000 hours. Factor in the 16-17 years of gameplay that wasn't tracked and I easily have over 10,000 hours.

I'm not sure I'd want to know the exact hours even if I could.

Most definitely continuum/subspace. It's been a long time, but I think the last time I checked it was around 16k hours. That said, it had a chat client that you usually left open in the background. So how much of that is actual play time I don't know.

Minecraft, played since beta 1.5 (sometime like 2011 or 2012 ish). I probably have 3k or 4k hours in it

Terraria, cant calculate entirely because kid me pirated the early versions but I'd guess 2k or so between vanilla and modded

League as well unfortunately. Met some cool people from it through my 3k or so hours

The only other game above 1k is TF2

I'm with you in Runescape. By far my most played game since coming back to osrs in 2018.

I keep taking breaks, but always come back. Many periods I've played more than my full time job, and I still haven't done nearly everything.

I'm not sure which one I have more hours in, but it's either Mechwarrior Online or the Mass Effect trilogy.

GTA 5 (online)
Both editions on PC amount to 2300 hours

Outside of that:
Mario Kart 7 on my 3DS
Mario Kart DS

Terraria

1055 hours!

Dota 2 is the most I have logged at around 3000 hours, but I don't have any friends to play with and don't want to deal with randoms, so almost all of it is against bots. Even though the bots are garbage, there's still so many creative, silly ways to play that game and ridiculous builds that are fun to try and see if they work.

Super Smash Brothers (64 and melee) is probably the most of all time though. Any time I pick up a new 2D game, I find myself comparing it to SSB in terms of mechanics. The way you control your characters in that series is the most innovative and compelling concept I have ever seen in a game. If I ever have the time to make a game myself, I would probably heavily model it based on that control scheme.

I also have to give a shoutout to Streets of Rogue, which is a fairly obscure rogue-like that deserves a lot more attention IMO. Another very replayable game with many fascinating play styles and ways to complete missions. It's on Steam and it's fantastic, I would highly recommend it.

Enter the Gungeon is another game I have high playtime on. It took about 40 hours before it started becoming fun, but man, it's awesome once you finally figure out how to properly dodge enemies and make the most of your weapons and items.

Steam says, out of those I have in my library there, Rocket League. But it's relatively closely followed by GTA 5, which I've also played on a console (those hours wouldn't be counted there), so I suspect that that's the real answer overall. GTA 5 is just a wonderfully deep game with an entertaining story where you can almost never run out of things to explore.

It's possible that Pokémon FireRed or Emerald has even more, that was too long ago for me to know for sure.

Spelunky*

Doom 2. Played since it first came out and played tons of mods

All in all probably Overwatch. It recently overtook HoI4 in most hours in my Steam library.

Fallout New Vegas at 200 hours

Not sure which game individually, but I have about 9600 hrs in SteamVR

For me it was when i was in high school, the space autism got me and i played something like 4000 combined hours in space engineers.

I don't even like that game.

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Well, according to Steam, the answer is a tool that used to be available to adjust GPU fan speed. At the time I had a video card that wasn't letting the OS adjust that, or maybe I didn't have the right driver, I don't really recall enough details to retroactively troubleshoot it.

But if I didn't use that even the least demanding game would very swiftly overwhelm the card because it didn't have any heat mitigation, so any time I played any game I would also open this tool and crank the fans to max so I didn't have to pay attention to them (I had headphones). It also wasn't that uncommon for me to forget to close it since it ran in the background once configured. In fact, it looks like I have even more hours in that than I do in EVE Online.