San Andreas by far. The game had so many elements that made it a classic. Most importantly, it's the only GTA game I've played where I actually emotionally invested in the characters, and that's important for any good game. It was actually rather sad when the game was over and there were no more missions to do.

For me, the story was the best of all the series, the characters were the ones most relatable, there were almost endless things to do, a huge map with a good many myths to explore. The actual setting of the game was great, if you're like me and like the whole West Coast feel. Good number of vehicles to drive, good music to listen to, it was quite enjoyable to listen to "Hellraiser" when off on a mission, or listen to Pac or Snoop and Dre.

Can't beat that, honestly, no other GTA game even comes close, in my opinion, San Andreas all the way. Unless Rockstar can bring the SA feel back to the franchise, it's on it's way to a slow death.