Definitely Arkham City.
The thought of wandering around with the possibility of the whole city being blown to bits by protocol 10 isn't very appealing but aside from that fact, I would really enjoy gliding around and meeting villains from the rogue gallery. The best part would be stopping them because I wouldn't have to worry about dying permanently. All I would do is respawn after painful torture by Professor Strange.
My second choice would be Seattle in Infamous Second Son, granted that I would be given powers too or else it would be boring having to just walk around a normal city.