It depends entirely on opinion, most people, think San Andreas is the best so far and the best game ever ( I would agree) due the massvice improvements over Vice City, such as climbing over walls, more aircraft, new vehicles, precision aim with every weapon and of course, Swimming, which many people thought the inability to swim in III and Vice City was bad, the environment and car handling was definetely better in San Andreas.
When it comes to graphics, it is entirely from perspective, Some thought the graphics in San Andreas sucked (I wouldn't agree, I thought they were great) because of constant orange colours, the weather is great though, I personally thought the graphics were fine, But I personally think that Vice City and Liberty City Stories had better graphics, Vice city may have been blurry for some but were good, and LCS's graphics, are amazing on the PSP, I have not played the PS2 version, I have owned the PSP version for 2 years now and it would not feel right to switch to PS2.
The graphics in GTA IV, seem to get mixed views, most people think they look great, I do aswell, the great brilliant graphics are not surprising, since the game is on next gen consoles, but might not be on PC (let's hope it comes out for PC) while many people do think the IV graphics are bad, but not everyone cares about graphics, it's mostly about gameplay, as long as they are good, we don't want it to look like pixel city.
Gameplay, between SA and IV, is different, San Andreas has nearly every feature everyone wanted, and after San Andreas, you are likely to have high expectations for the next games, Player's views of IV were sometimes swayed when Rockstar announced that features from San Andreas and other previous GTA title's such as Flyable Airplanes, a country and desert landmark and Pay N spray's will not return, I find the "no planes" part disappointing, Rockstar say there will be no planes to make it more realistic, which is weird, though it is possible that R* are removing planes from IV just incase they remind people of the September 11,2001 incident.
IV is still not out, so I don't know what to decide, even if it is actually better in someway, despite old features being absent, San Andreas will not immediately lose it's popularity with fans of the Series.