The safe bets have already been mentioned for RPGs on the PS3. You're sure to get a great experience from the Tales games and Ni No Kuni, FFXIII and FFXIII-2 are good as well, though they're going to be polarizing depending on the individual. If you missed out on KH, it is available as 1.5 HD with Chain of Memories and 358/2 Days. The lesser known RPGs are going to be more hit or miss depending on your taste. There is Disgaea 3, 4 and D2 available if you're up for Tactics style RPGs. Dragon's Crown is a great side scroller RPG. I would highly suggest the Atelier series, but they are going to be a different pace from what many are used. Atelier Ayesha is a good beginner game for new players to the series, but Atelier Totori is the best for story and Atelier Meruru is the best refinement, though Ayesha has the best combat until Escha and Logy comes out next year. Hyperdimension Neptunia Victory is the best of the series for entertainment and combat. Demon's Souls or Dark Souls are great games if you don't mind the challenge they present. Resonance of Fate is a really fun game with an interesting combat system, the main story tends to lag until late game, but the character story is interesting. Star Ocean 4 is available if you're want your next Star Ocean experience.

And because it deserves a separate note for it, Valkyria Chronicles is one of the must play games on the PS3. It is sorely under looked, but a beautiful game to watch and just as enjoyable to play. Find a copy and play it.

If you want more choices let me know, I've played most of the RPGs on the PS3 and own all of the JRPGs on it.