According to Steam, Arma 3. But I know for a fact that a lot of those hours were not technically in game because I would spent hours upon hours in Notepad++ scripting for my modules while I had the in-game editor running in the background to alt-tab in an test things every now and then.
TF2 is second, and that’s all actual play time, baby.
However, in Ultima Online I spent a good 8+ hours a day, every day, from 1997 to 2003. So probably that; I just have no actual counter for the time put in.