GTA V. In terms of it's technical excellence, graphics, and not having a wimpy little bitch as the main protagonist, certainly.
SA was fun, but it really hasn't aged well. That giant breast they refer to as a mountain is a complete joke. And SF feels completely unfinished to me - like they realised they had one 3rd of the map they needed to fill and just put a bunch of straight-line streets up it.
I have never played a game with a map as realistic as V's, and for me, the map is pretty much the main draw - I'm an explorer. I booted up last night and spent an hour walking around before doing... anything. It's that good, in my opinion, that I could probably spend a 3-4 hour session just wandering around without doing anything. Listening to peds, hopping walls and fences and checking out the people by the pool, walking up and down the beach. It's glorious.
In SA, I could probably manage an hour tops before I got bored. I replayed it in full just before V was released and it was fun, but ultimately felt dated, empty and rather dull by comparison.