My favourite so far has been San Andreas, closely followed by IV, then III and lastly Vice City. Reasonings as follows...
San Andreas' story wasn't the greatest and as many others suggest, it was not realistic. However, this game had the most replayability out of all of the games, it was so vast, and the fact we could get away from the city life was a breath of fresh air. I played this game a lot, and I enjoyed it.
GTA IV is great, there's no denying that, but i've only just started to play it, maybe it will rank above SA once i've had a real fuck about with it and completed it, but until then it closely follows SA. This game seems much more fun what with the physics anarl...
I placed Vice City last, probably joint third with III. I wasnt really old enough to appreciate the 80's vibe having missed out on that decade- music was still great though...