Vice City was the best, it had loads of personality. Best music, vibe, funky coloured jackets and the coolest "I've hit the bigtime" moment at the end when you get the mansion.
SA was also great, but it was great for the size of the map and varied locations. Had some great characters too.
IV was bland to me. Characters weren't really interesting, city was "samey", there was no "fun" factor that the rest of the series had. Never felt attached to it really.
I do wish Rockstar would sort out their PC development. I've really gone off consoles this generation, even though there are some great games availible, I just prefer the PC so much more.
I would do anything for a great port of Red Dead Redemption.