As much as I hate to say it Vice city. I'll explain
I loved 3. But once you play the other games after it, it no longer seems special at all.
As for San Andreas, It just suffered from being too damn long. I got it when it first came out, I got bored and never bothered finishing it until this year. And the whole defending gang territory was just frustrating, amongst other things such as taking damn near ten minutes just to drive to a location. There was just too many things I didn't like and it over shadowed the things I did like.(I hope they never put planes in another GTA game again)
GTA4. *Sigh* I don't think this was the worst GTA game like most people do.(these people obviously never played GTA1&2) But it failed in a big way because if you're going to have someone playing a game for 10+ hours, at least have some fucking mission variety. It felt like I was doing the same thing over and over again. That was pretty much my only issue with this game. Unfortunately that is a very big problem. It would have been my favorite had they just added more diverse missions.
Vice City had everything going for it. 80's setting, fictional Floridian location. Ray motherfucking Liotta!. But the story was terrible, and predictable.
There wasn't a single thing in this game that I didn't see coming beforehand.
personally I wanted to throw this game in the trash after beating it for being so damn predictable.....But Ray and the 80's prevail over the stupid failure of a story that tries too hard to be a Brian De Palma flim.