Looking at my Steam games list, my top 5 when it comes to playtime are:
Trailing not too far behind are Civ V at 196 hours, and not included because I don't have it on Steam is Fallout 3, which, if I add the playtime of all my characters, is at around 700 hours. Dark Souls 2 is one of my favourite games as well but I've lost track of playtime on that one.
I don't just spend hundreds of hours on certain games though. I tend to play stuff like Isaac or Terraria a lot because I usually watch videos/listen to podcasts or music while I play, but right now I'm also plowing through Shadow of Mordor (possibly my favourite open-world game in a long time because I was expecting a mix between an Assassin's Creed clone and a Arkham City clone, but it beats both games IMO).
If I had to recommend a weird game series that deserve more fame because of the ungodly amount of fun they've broguht me though? The Half-Minute Hero series. The first one is decent, the second one is fucking amazing. The whole principle is that it's a series of levels in which the world will be destroyed in 30 seconds and you have to level up your hero from level 1 to a high enough level to beat the boss. There's always a trick so it's really more of a puzzle game. The second one has a ton more role-playing elements to it, a good story that actually makes you care for its characters who go from cliché one-dimensional JRPG characters to fully-fledged people, some really funny dialogue, a fantastic localization, some nice pixel art and some amazing music for a game that looks that silly.
Fuck it. That was a big tangent, but now I'm off to playing it again.