I'm going to say Online. After I finished the story, I couldn't really be bothered to do much more of activities available except for rampages and a few hobbies etc. I will definitely finish everything off in the SP eventually, but the Online is pretty much endless havoc. Even though it was a launch failure, and with the current modders and all..the game can still be immensely enjoyable.
Signature by Mr Chaotix
CURRENTLY PLAYING: THE CREW