Excluding MMOs, my most played game is Path of Exile, with roughly 1100 hours.

I also played around 500 hours of Torchlight 2. For me, ARPGs are a great pass-time.

MMOs:
World of Warcraft: 2500~ estimated
Final Fantasy XIV: 1200~ estimated
Final Fantasy XI: 300~ estimated

Longest running in years, I've been playing the original Legend of Zelda since 1988.  It's not a very long game, so no idea on total hours I've put in over the years.

Other games I've played multiple times and have repeatedly gone back to for uncounted hours over the years:

PC

Crusader Kings
Star Trek:  Birth of the Federation
Europa Universalis:  Rome
Making History:  The Calm & the Storm

Back before my friends and I all got married, we used to get together once a week and play sports games.  As many as 6 of us would get together and play seasons of football controlling different teams, or creating wrestlers and playing out careers.  So during that time I've also logged a ton of time into:

Tecmo Super Bowl - NES (mainly 1990 - 1998 until Franchise mode became a staple of the Madden series)
Raw 2 - Xbox (played heavily 2003 - 2006)
Smackdown vs Raw 2007 - Xbox 360 (General Manager mode made this our staple wrestling game from 2006 on)
Madden 13 (Wii U)  <--- I've gotten over 4 years out of this game since EA decided never to release a football game on Nintendo since.  My best friend still does come over to play this one.

Most likely Starcraft. Played the campaign, user created scenarios and lots of LAN-parties.

But I cannot sure, as I have no hard number. High in the race are also: XCOM:Enemy Unknown, Doom II, Descent, Duke Nukem 3D, Pinball Dreams. Had more time to play as a kid - and less money to buy many games.

This thread returns to remind me of my 4000+ hours on runescape and my creeping up on 1000 hours on the guitar hero series

Splatoon is my life.

Hearthstone. Between 700-900 hours. Still playing although less so recently.

I wish it had a counter to know how much you've played. I've been playing since closed beta and I wonder how many hours i've put into it.

I have spent around 5400H total in Total War games
- Empire Total War: 1600H~
- Napoleon Total War: 2000H~
- Shogun Total War 2: 1000H~
- Rome Total War 2: 800H~

This is what I can see logged in Steam, I can even imagine how many hours i've spent on pre-steam Total War games.

Yea it is an educated guess. Playing a little everyday adds up. Those people who go for high ranks every month probably are putting insane numbers. Watching Hearthstone tournaments is fun too. Just not the silly Jade meta. 

Bet nobody can guess mine.