I need another addictive game

Hello there and thank you for taking the time to help me with my dilemma. I have over 1000 hours on Garry's Mod because of its versatility in game modes but I can't seem to find another game like it... Can anyone recommend a game that has an addiction factor associated with it? Thanks and personal experience is the best source for knowing about the game. Another way to word this question is: What game are you addicted to and why?