1. fallous new vegas - amazing story, and some nice dlc's aswell, the main storyline, dlcs and side quests can easily lead up to 300+ hours, no filler dlcs like fallout 4 and some rushed up story, in fallout new vegas you can be a psychopath who chooses for him self, you can join up the legion as a cool roman, join up the ncr and even join up with the brotherhood of steel, you are bound to a main story but can decide the outcome yourself in so many ways, different from fallout 4 where "you are the COOL father who tries to fight the bad people and get his son with filler dlcs"
2.gmod - so many things to do, you can have fun with your friends which can lead up to +1000 hours (+3500 in my case), met some amazing people who turned out to be really good friends aswell
3.left 4 dead 2 - used to play this game with my brothers back in 2009/2010 on the xbox 360 and it was such an amazing experience, convinced my ybrothers to buy it on the pc aswell and we all loved it, many hours were taken by trying to survive in survival mode and trying to complete campaigns on expert mode, and ofcourse playing versus
4. team fortress 2 - even if the game is pretty dead nowadays, i had pretty good experiences