GTA 4, by far. After i didn't like San Andreas all that much i was pretty happy that it started to, for the lack of a better word, grow up and do something more serious for the sequel without actually completely losing it's humor. Sure, Saints Row 2 was pretty decent but it's just not in the same league. GTA 4 had a dramatic story and hilarious satire and i appreciate that. It has also much better mission design than the Saints Row games.