Honestly it's probably Saints Row 1 and 2. And I'm not entirely sure why. Any GTA-like is fun to just muck about in and cause a rampage with, but for some reason in those games its more about going everywhere and doing everything. Somehow I just get absorbed in the junky and amusing city life, with all the weird comments the civilians make. Sometimes I just drive around. Listening to the music, enjoying the in-game rain, or the weird commercials and radio broadcasts. I follow ambulances and fire trucks to see what kind of hectic activity they're up to. I hunt down weird alien things in the ocean or join races or fight clubs just for the lols. It does something for me that GTA doesn't for some reason