I'll go with Vice City, simply because it never took itself too seriously, and it kinda perfected the formula from GTA III.
San Andreas had the coolest features, but the story was another bust. The Housers really don't know how to make their clichés interesting.
GTA IV was terrible in most ways, except for the tech. I was pretty sickened when I shot a person the first time, because their reactions were realistic rather than stylized. It looked like it hurt. Sorry imaginary dude.
That's really the reason I'm picking up V. I'd be very surprised if the Housers suddenly came out with a great story, but I really want to see the tech.