Oh. Don't forget PSIV! It has an unfair advantage by being way newer than PSII, but I'm going to have to say PSIV is the best RPG on the system. Damn that was an enjoyable game... especially after playing it right after finishing PSIII... the speed the character walks in PSIV was enough to get me excited after PSIII. (Not that PSIII was that bad either.)
PS4 certainly is a more polished game, and it's definitely aged better than any of the other PS series. Absolutely a great game. That said, I think I beat the whole thing over a weekend without much challenge. Doing that with PS2 (*especially without the hintbook*) would take some doing. Perhaps because PS4 was much shorter, it didn't leave as a big as impression on me as PS2.
PS3 was good, but it just didn't have the "PS feel" and the mood while playing the game just feels out of place compared to others in the series.
PS1, what can I say, that game was at way, way ahead of its time. In 88-89 -- seeing those dungeons, big enemies, colorful background, animated fight scenes, vehicle riding, that sci-fi overworld, etc, it blew my mind back then.