1k+ Hours Recommendation!

What is that one game that you just can't stop playing? What makes it your favorite? Why would you want to recommend it to someone?
I have been running out of things to play and I have scoured through the steam sale, with little to no luck. I am looking for a recommendation for a game that you guys have enjoyed and loved for a very long time, and or mainly played that game the whole time!
I am open to all suggestions, i love all games, especially if it is a good one :) Thank you in advance!