GTA:San Andreas is still, imo, by far the best GTA game to date. That game was so awesome I bought it twice and probably spent at least 1000 hours on it, finished it some 5 or 6 times (only 1 or 2 times were done using cheats), each time doing something differently or trying a new path through a mission. And, I think, the story in SA is better than GTA4...which left me feeling rather...empty? Ok so you kill the dude and now what? Nothing else happens, y'know?
At least in SA you finally kill Tenpenny and Big Smoke (after you found out what all the subtle hints about him not being loyal to "The Grove" throughout the game) and can get on with life in the hood.
And GTA:SA was awesome fun to cruise around on because A)It wasn't totaly impossible to navigate, by the time I'd finished the game the first time I knew exactly how to get anywhere from anywhere, on GTA4 I couldn't even find a gunshop without looking on the map and using GPS. >_<, and B)You had the car customization thing, which although a bit NFS was pretty cool.