Didn't play Godfather (it's on the backlog pile). I always seem to burn out on and never finish a GTA game. Same for Crackdown, it was fun for a while but for some reason I got bored. Mafia was great, but I don't really consider it an open world game (I don't recall there being that much to do outside the main storyline).
That said Saint's Row and Saint's Row 2 for some reason really clicked with me. I played both to the end and had a blast doing so. I loved SR1. SR2 improved on the first and took it to another level. Really for me, nothing else comes close.