Time to show off my lack of life!
1) Mount and Blade: Warband - 1,255 hours
2) Crusader Kings II - 1,087 hours
3) Europa Universalis IV - 1,079 hours
4) Civilization V - 509 hours
5) Victoria II - 218 hours
6) Hearts of Iron IV - 216 hours
7) From the Depths - 174 hours
Stellaris - 169 hours
9) Sid Meier's Railroads! - 163 hours
10) Medieval II: Total War - 99 hours
Not included: I've never actually played Gary's Mod but I opened it once and due to some weird glitch the time kept ticking, thus how I have 104 hours.
Also: Do I win?