GTAIII was really hard. Claude's flexibility was very limited as you could not rotate the camera while running/shooting etc without stopping. The missions were also pretty hard because the cars blew up after a few bumps.
Vice City was just as hard. And if not, harder.
San Andreas was too easy. I loved how you could finally freely rotate the camera. The only thing that made San Andreas so hard was like what Thomas said, there were so many options ad side missions to do that it would take at least 150 hours to complete. The highest I ever got in San Andreas was 92.1% but thats was deleted because I sold my Xbox