I'm 3 hours away from 1,000 on Arma 2 OA, but Blockland has like 1.75k
List:
1: Blockland (1,741 hours)
2: Arma 2 OA (998 hours)
3: Kerbal Space Program (393 hours)
4: Garry's Mod (231 hours)
5: Rust (132 hours)
6: From The Depths (123 hours)
7: Skyrim (119 hours)
8: Arma 3 (69 <3)
9: War Thunder (64 hours)
10: Cities: Skylines (48 hours)