Most hours spent on one game?

Definitely World of Warcraft, I don't have a subscription anymore to check my ingame played time but I reckon all summed up it's close to 200 days or so

Almost a year of my life playing a game, lol

I spent 2000+ hours on League of Legends although I don't play it anymore

CS 1.6, more than 4k hours. Without Steam even more.

Played RS since I was like 8 so wouldn't be surprised at 5k+ hours. Also have around 3k on CSGO

I Have roughly spent 5042 Hours on Garry's mod sadly

17 straight hours went on WOW back when WOTLK was around

Pretty much just gta. Lol Ive spent around 400 hours

7 days to die with over 1000 game hours+
and it's still in alpha

I have invested over 3500 hours into warframe and don't regret a single second.....aside from leveling the plasma sword

Garry's Mod with 8500 hours... I think my next closest game is Payday 1 with like 300.