III was certainly revolutionary when it came to open world play, and changed the face of gaming. But, it's not the best game of the series...
For me it was a hard choice between Vice City and San Andreas. I went with Vice City...mostly due to the music and plot (I mean, what 80s kid doesn't want to play around in the world of Miami Vice?)
It did have some disappointments though. Like, the cessna at the airport with it's clipped wings.
San Andreas was one that I enjoyed emmensly, and I think had the best plot of all three to that point.
IV disappointed me. I could never get into Nico, though I thought the spinoffs (ballad of gay tony and Lost and the Damned) were pretty fun. I just couldn't get into what they'd done with the new game engine (RAGE)...during that period though, Red Dead Redemption came out and is hands down one of my favorite games (up there with Vice City and San Andreas).
Now I've got V, and I've been playing it for a few days (can't go on a marathon...these bills won't pay themselves!)...It's great. Awesome story, interesting characters. So far, I love it. But, it hasn't captivated me the way the Vice or SA did...
Still, a huge step in the right direction. Maybe my mind will change by the time I finish the game.