PSI, Perhaps the best 8 bit RPG, you won't do very much leveling up, as the max level is 31. The battles are fun and the dungeons are the clincher for this game. Nowadays RPGs are too easy get through and around, and it doesn't feel like you're exploring, just following a set path. In PSI you have to pay attention to where you're going so you don't get lost.
I agree with sizone on PSII, it has a drawing atmosphere, with alot of backstory. You'll be doing of fighting for experience... but you won't be doing very much leveling up, but it'll feel so good when you do.
PSIII has very good story, I think, but i think that killed it for me, was the lack of continuity, not with the story, but the technicalities. "Cyborgs" that can be healed with techniques?? Also, even though the battles are fast, there are hardly any animations for the enemies, and when you're fighting, you just don't feel like you're fighting.
Alot of people love PSIV over the rest. It had better graphics, and a continuously developing plot that was more notable because of the cut scenes. The battles are really fun and hard, esp in the air castle.
Perhaps one of the best thing about the series, is that they are all related, unlike Final Fantasy. Alot of the unexplained and mistranslated yields to alot of fun debating among PS dorks.