Volition has proven themselves to be the masters of making open-world games fun, so they've already got my money for this.
This. SR3 was so good, only open world game I actually finished.
Thinking about it...I think this may be the case for me as well. I didn't play SR1 (didn't like the demo), but I played the shit out of SR2, SR3, and SR4. SR4 wasn't as good as the other two, however, and then Gat Out Of Hell was so mediocre that I quit playing it. SR3 was the best one, easily.
EDIT: Crackdown! Forgot about that game. Played through that, too. Have never finished a GTA. They always got boring so quick.