I'm probably going to be playing csgo because I have literally nothing better to do. At least, not that I feel like doing! If I get bored of that most likely heavily modded Fallout 4.
By far my two most played are Garry's Mod and Counter Strike Global Offensive at 1,533 hours and 1,500 hours respectively.
The three under those are Black Ops 2, Team Fortress 2 and Terraria. Those being at 686, 439, and 230 hours played. Quake Live would probably be higher than Black Ops 2, but...