San Andreas for me every time. The start of the game especially, when your a hood working your way up, that just felt like a 90's movie like Boyz in the Hood. The music was great too, besides the expected gangster rap we had Faith No More blaring through the radios, what more could you want!. The introduction of bicycles was great too, and the Sanchez - I spent more than 40 hours just trying to beat my wheelie records and do mad jumps.
Ohh and the parachute! - get a bicycle up on a roof and jump off and parachute down.
Ohh and the casino's were cool as well.
Yeah GTA:SA had a lot in it, even the 2-player mini game thing was pretty cool. I'm just itching for a 360 GTA now, should be some serious gameplay in that. Imagine if they do what Crackdown does with the game hosting, even playing with 3 other players would rock.
We're going down... in a spiral to the ground...