I would've said GTA V, but it's unfair because obviously now they have more understanding and opportunity to make it better. I put Vice City because it's the first one I got when I was... 8, but that's a long story.
Vice City. It's the first game i played of the franchise. I was even upset when they anounced GTA5 would take place in Los Santos rather than Vice City. Anyway, hopefully GTA6 will take place in Vice City.
I think Vice City was where the series started looking for the right direction, San Andreas was where they totally found it, IV was where they totally lost it and V was where they started to look for it again.