Not a steam user personally (thank you for standalone launcher!!) but my Top 3 are…
1 WoW - 1 character with literal years of playtime
2 D2 - 10,000+
3 PoE: 10,000+
Last Epoch is currently making a run at dethroning Diablo 3 at #4 then eventually eyeing top 3 
Wow! That’s the funny thing about games. I remember being so stoked about HommIV coming out, and even HommV, but buying them and being sorely disappointed. At the time I was unaware of the phrase “jumping the shark” but that’s totally how it felt.
Similarly, I never really got into the Civ games, and most people I’ve met either preferred Homm or Civ. I did like Age of Mythology quite a bit though.
Yeah, I had that with MOO 1 & 2 (which I loved), then MOO3 was released. 
They’d managed to make it less enjoyable to play than a spreadsheet (& I’m an accountant)…
Well homm IV was the first game of the series i played and there are some features in there i just can’t live without! I first got to civ games at civ V and i liked it okay and civ VI was decent as well.
HoMM IV is good on its own especially taking into account community patch for it.
P.S: I would recommend to take brief look at HoMM II… 
eso, skyrim, and warframe.
Steam says I have ~4.6k hours in Path of Exile, but I have moved to the standalone client a while ago… so I assume I’ll be around 5.5k hours there now.
Had to add 5 so LE would be on the list. 
Well this is a fun list, so I’ll throw mine in there too. Kind of a fond trip down memory lane in a way.
Total War Warhammer II: 1,253 Hours
Grim Dawn: 822 Hours
Last Epoch: 481 Hours
With PoE coming in a close 4th at 423 Hours. Most games are in around 140-200 hours. Despite only about 200 hours, I would have to say Witcher 3 is probably my favorite game in my library. I only played it through once, though, but man what a game.
Mind you, this is only Steam. I had by far the most hours in WoW (raided all the way from vanilla through to Lich King, before I stopped playing it for good), and D2 is probably right behind it. The countless hours of boss runs that I did in D2 just can’t be counted, and that was a game I actually played with my dad and brother too, so that gave it a bit of extra mileage as well. Cow Level farming, moo!! Too bad Activision bought blizz, and pretty much devoured its soul.
POE: 2,283.6 hours edit: actually 3,283.6 misread lol i got no life
Torchlight 2: 400 hours
Divinity original sin: 200 hours
Disgaea 5: 168 hours
Bioshock: 100 hours
I got standalone LE client so ill guess at least 100 hours
Mine are
For me it’s :
I don’t play much on Steam. Out of this, my most played games are Diablo 3 (over 2000 hours), Overwatch (unknown duration), StarCraft 1 (solo) and 2 (solo and cooperative). And in the past, mainly the whole “Age of…” series, the three WarCraft, the whole “Might & Magic Heroes” series.
My most played games ever are probably D2X and Morrowind though, but not on Steam.
On steam
PoE 2,4K hours
GD 500 hours
TQ 110 hours
Out of steam
WoW (probably most hours of any game for me)
City of Heroes
D3
Hi
Mine would be:
Borderlands 2 - 401 hours
Skyrim - 251 hours
LE - 146 hours
Destiny 2 - 118 hours
But non-Steam games by FAR the highest -
Diablo 3 - 892 hours
ElderScrolls Online - 732 hours (Non Steam when I first bought it)
Keys
sadly the 1. is PoE because the 2. isn’t anymore 
Marvel Heroes 3. GD
Grim Dawn - 874h
The Elder Scrolls V: Skyrim - 403h
Borderlands 2 - 302h
Last Epoch - 220h
-------------------------
POE - 26h
Divinity Original Sin 2 : 322hrs
Might&Magic X Legcy : 274hrs
Last Epoch : 101 hours
Obviously PoE would be wayyyyyyy ahead of all these if I used it via steam, but I don’t.
The game I played the most is still Everquest, though (Around 800 days played I think? Hard to say since I have the days played for each characters but some characters were boxed).
I don’t really use steam even though I have around 250 games on it (major backlog lol). If I am counting based on total hours it would go…
3) Path of Exile - 4kish hours or so, many afk
2) Diablo 2 - Played this for like 4 straight years easy
Path of Exile - 1114.41 hours
Rocket League - 892.02 hours
Destiny 2 - 564.54 hours
Back in the day… World of Warcraft , Call of Duty 4, Diablo 2 probably had years worth of game time.
Path of Exile - 3993 hours
Warframe - 752 hours
Neverwinter - 278 hours
Last Epoch - 40 hours… which isn’t that bad since I bought the game on March 6th, but didn’t have too much time (1-2 hours a day) to play until the 12th 
I have a feeling LE will climb the ranks quite fast.