Definitely San Andreas. I have collected this game for as many systems as I could, and even got the roms on my computer. I remember playing this game so much, I still do. I stopped playing GTA 5 after I beat it, but even after so many years being out, I still play San Andreas as if it's still new.
I don't really care about the graphics at that time, and even now. It's was really all about the atmosphere, plus cheat codes make the game super fun.