According to Steam, Killing Floor is my most played game with just under 300 hours. I like Counter Strike Source is in second place with 200 hours. It's funny because I didn't really think I'd played those games all that much compared to some others. I would hazard a guess though and say that Battlefield 2: Modern Combat was my most played game; I clocked over 500+ hours on one of my accounts.
Currently I've been playing a lot of Borderlands 2 with my brother. :D
Terraria is my most played game on Steam I think I've played other games for far longer though. Games like Spyro and old ps2 games lasted me for hundreds of hours.
The game I play the most is Mario Party on the Nintendo 64, I really enjoy that game. My boyfriend, his cousin, and I usually have a few drinks and play the game, it makes things more interesting and a lot more challenging hahaha.
I played Minecraft nearly every day for 2 years, and still play occasionally. Lately it's been Borderlands 2...I have 250 hours logged so far, and will probably get to 400 before I get tired of it.
Has to be the fm series on pc clocked just shy of 2820 hrs alltogether from fm 11. However on the console's will be most likely bf I hate CoD with a passion.