I'll give you outdated playtime on Steam.

Team Fortress 2
76 hrs on record

DOOM 3: BFG Edition
75 hrs on record

Call of Duty: Modern Warfare 3 - Multiplayer
66 hrs on record

Retro City Rampage™ DX (It was AFK, i admit)
44 hrs on record

Call of Duty: Modern Warfare 2 - Multiplayer
42 hrs on record

Garry's Mod
27 hrs on record

Deus Ex: Human Revolution
25 hrs on record

Wolfenstein: The New Order
21 hrs on record

Far Cry® 3 Blood Dragon
21 hrs on record

Left 4 Dead 2
20 hrs on record

BioShock Infinite
20 hrs on record

Battle.net:

WoW
4 years on record

Diablo III
21 hours on record

Non-Clients(pre-Steam):

Doom
19 years on record

Quake
18 years on record

Deus Ex
16 years on record

System Shock 2
13 years on record

Quake 3 Arena
8 years on record

1) Hearthstone - Approx 450 - 500 hours

2) Heroes of the Storm - Approx 100 hours

3) Diablo 3 - 50 ish hours

Highest on Steam is Borderlands 2 with 28 hours. 

Well, I had osu! at more than 2000 hours on Xfire when I stopped using Xfire to log my playtimes... And it's been a good while since (apparently the Xfire I knew even shut down months ago) and I've played it a lot after that. I think I also had both Wolfenstein: Enemy Territory and Enemy Territory: Quake Wars at over 100 hours on Xfire, maybe even over 200 hours. And that's not including me playing Wolfenstein before I started using Xfire.

As for Steam, the top games are like this:

1. Sid Meier's Civilization V: 84 h
2. The Elder Scrolls V: Skyrim: 75 h (not including my time with a less legal version of the game)
3. Total War: SHOGUN 2: 72 h
4. METAL GEAR SOLID V: THE PHANTOM PAIN: 66 h
5. SimCity 4 Deluxe: 52 h
6. Cities: Skylines: 39 h (I think I should play this a bit more, it's almost shameful this is still below SC4)
7. Grim Dawn: 34 h (I think I've owned if for about a week only!)

Hmm, Grim Dawn reminds me of Titan Quest, which I've definitely played a lot too... Anyway, of the above games, I've completed every one that can be completed except for Grim Dawn, which I'm currently playing very actively and don't see any way I'm not completing it very soon. The overall top 3 is probably this:

1. osu!
2. Titan Quest + the expansion, Immortal Throne
3. Wolfenstein: Enemy Territory

Also, for the poll, my favourite platform is GOG. I really dislike Steam because it's cumbersome to use (especially with the semi-forced mobile authentication if you want to use the marketplace) and especially because it's essentially DRM (I know it's not, but most games seem to use Steamworks as DRM, and it's not easy recognizing and obtaining DRM-free games on Steam). For me, GOG is the only platform I actually like, Steam is barely OK, and everything else just plain sucks. However, roughly evenly with GOG I rank other DRM-free stuff (including DRM-free physical games), and other platforms come only after that. Yup, DRM is a pretty big deal for me. I disagree about the principle, and I think it's going to cause problems in the future (some forms of DRM already have).