What game have you played the most?

1 - Path of Exile with over 3-4k hours (steam only has 1579 counted as I play mainly on non-steam client since steam takes ages to download a 10mb patch)
2 - Skyrim with probably over a thousand hours played (again steam only has like 30 hours registered as I started playing on a pirated version, then bought the game and never bothered moving my save and mods to steam just kept playing the pirated version lol)

I guess games I have a similarly high playtime on CS 1.6 and WC3/SC/D2 but those were so long ago that I'm not sure if it's just my imagination making me think I've played them for so long.

minecraft

Borderlands 2
317 hrs on record

Borderlands: The Pre-Sequel
142 hrs on record

Fallout: New Vegas
117 hrs on record

Counter-Strike: Global Offensive
113 hrs on record

Borderlands
109 hrs on record

Left 4 dead 2 - 1,731 hrs on record

Garry's mod

EQ2 - thousands of hours (and account sold for $2500 when I quit).

On Steam, Borderlands 2 with 930 hours.

On steam, Town of Salem, It's the only gave I have over 200 hours on. 217, I also played a lot before I bought the game on steam. Spent a whole summer playing almost all day every day on browser. Closest behind that is Gmod with 168 hours

In general probably Minecraft, don't play it anymore really. But for about 2 1/2 years, that was nearly all I played, I'd play for hours.

Warframe with 731h :x

Top 3 (because I'm ashamed of Dota 2, need to put more hrs in M&B or Dominions 4)

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Wargame Red Dragon 900h

Still TF2 at 3,347 hours, even though I haven't played it in... years... I don't know how long it's been -- only a handful of sessions once the Mann vs Machine update came out. At least half of that is not idling, and the hours are actually higher, since I was playing it for a long time before Steam tracked hours played.

Clicker Heroes is disappointingly the runner-up at 556 hrs, but at least Borderlands 2 (361 hrs) and Defense Grid: The Awakening (341 hrs) follow before another clicker shows up -- Adventure Capitalist (204 hrs). I'm still playing Skyrim, so at 171 hours currently, it has a chance of at least surpassing one of those.

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Payday 2 - 683 hrs (dead game, fishy dev)
MWO - 571 hrs (BT ftw! but fishy dev again)
ArmA 3 - 434 hrs (x100 for it´s predecessors off steam)
Race 07 - 226 hrs (finally something to be proud of!)

Looking at that list i see how i´d play lovable games more if they were´nt made by moneysucking [redacted] :<

From what I can see it's Warframe with 1319 hours.
But I spend 5 years playing EVE-Online of which 3 years with dual accounts, but there is no way of telling how many hours I spend on that game. And also spend a ton of time in World of Warcraft.
And last but not least I been playing Hearthstone since beta every day, but again, not sure how many hours it totals to.

Guild Wars 1 and Diablo 2 lord of destruction (non steam) probably more than 4K hours in each.

Orcs Must Die! with 925 hours. ^^

Non Steam? Without a doubt WoW...played it far too much xD

But my top 3 on Steam are funny...especially the first - because nearly no-one even knows it once was distributed via steam in some regions^^
But I had way more hours in that game...steam never added their upgraded Launcher and it was unplayable via steam since then.

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I've played so much Warframe that, at this point, I think no game on Steam will ever surpass the hours I've put into it.

Battlefield 2: I'm sure I've got 1000 hours on this

On steam: CS:GO, 334 hours.

Payday 2 with 902 hours

1. Killing Floor - 2,030hrs
2. Dead By Daylight - 1,146hrs
3. Rust - 934hrs

first two games are still counting, Rust I pretty much dropped for now^^

Skyrim / New Vegas, can't be bothered to check which is higher. One is 300-something hours last I checked, and the other (most likely Skyrim) is 500~.

CIV I - V
Dota 2
Duke Nukem 3D

Sanctum 2 with 170 hours

Dark Souls I, with around 160 hours. Followed by the sequel with around 110 hours. I don't play much....