Company of Heroes 2 (135 hours played) That is what steam says anyways.
However, that is not my most played game. Here is the real list with no numbers to back it up, but I know for a fact that these are the most played for me.
1. World of Warcraft
From the age of 13 -15 I spent every waking hour playing WoW. I have never been addicted to a game this badly before, and hopefully I never will again.
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2. Halo Reach
I actually had a real life friend during this time, and we would hang out at my house and play Halo Reach like no tomorrow. Plus, I was just addicted to Halo at this time. Unfortunately he stopped speaking to me for no reason and I never heard from him again...

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3. StarCraft
From the age of 10 - 12 I spent most of my time playing StarCraft, and it was thanks to all the cool custom maps you could play with people online.
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4. Spyro Year of the Dragon & Ripto's Rage
I love, love, love Spyro and I used to play it a ton along with Banjo Kazooie when I was younger then 10. To this day I will still replay the games. Also, A remaster is coming this year so booo ya!
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5. Sarges Heroes and the Army Men Franchise in general
These games were my first addiction to video games. I loved playing with those plastic toy army men when I was a kid, so it is not a surprise that I would fall in love with these games.
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So, what games have you sunken the most time into??