I ask you, and anyone in this thread, what is the most time you've spent with a game and what was it?
For me it's a tie between Skyrim and Civ5, both around the 200 hours area. I know that doesn't hold a candle to people who spend years on MMOs but for me that's a ton to put into single player experiences.
From the ones I can verify GTA V on Xbox 360 at around 300 hours, but I am quite sure I have put more hours during the PS1 era and before in games because I didn't buy that many games back them because of money constraints. So I would guess something like NHL 2000 as I did play that game a lot (I played more than 5 full seasons in that one). I never found a way to see the hour I have put into PS3 and PS4 games, so possibly a PS4 game, although I doubt it. Forza Horizon 3 seems to be 255 hours at the moment, so that one potentially will get to the nr 1 spot in some time (as I still play it weekly for the challenges and I still want to 100% the hotwheels DLC (but I am not sure it will happen or not as I don't like that DLC as much as the rest of the game or the other DLCs)).
edit:
I have also played a lot of tetris on the original GameBoy and GameBoy pocket, but I don't know if I went past 300 hours or not over the years. It is possible I did, but impossible to know for sure.
Also, now that I think of it, it is possible that I put a lot more hours into Counter-Strike 1.3 to 1.6 and CS Source back in the day, but steam didn't count the hours played back then (and with 1.3 Steam wasn't even a thing yet), so no way to know for sure.