I think I've put the most hours into War Thunder with World of Tanks close behind.
Come to think of it, Il-2 Sturmovik might lead the way for number of hours played. I havn't played it seriously in years bet when I was playing it I know I was always on it.
My most played game is LoL. I play it everyday and I think I spent countless hours on it. Hearthstone and Minecraft come close, since I enjoy playing those as well.
Other than that I also play Skyrim and GTA V from time to time, so I'll put thos on my list too.
At the moment I belive it's League of Legends. It has someting like 200k or 300k online users at a time only in the EU I guess. It's impressive how many poeple plays it and how many people actualy enjoy playing it. I tryed it and I generally get bored after the 2nd match.
My most played PC games are the Football Manager series. A lot of idling, sure, but I have 4 games in the series and the least played has 170+ hours in, the most played about 350+.
Other than that, it's Killing Floor with about 335+ hours and counting!
I wasn't sure it was League of Legends, but now upon searching again on Google, every trusted source can confirm this is the most played game worldwide, and it's understandable why... it's an easy and straightforward game, it doesn't require an advanced computer to be capable of running it, and it's overall noob-friendly in the beginning. They've really managed to make the most out of its potential since they launched this game in 2009! Good job to them for creating a recognizable brand out of a not-so-new concept.
Mine's between League and Terraria. League is so fun and playing friends is a great laugh, but Terraria has insane replayability and the updates every year make it an almost completely different game, I've played for maybe 1000 hours over the years, only about 100 with my friends.