Crisis Core is good if you like the world of FF7 and want to explore the backstory, in an action RPG.

Disgaea is great if you like tile-based turn-based strategy RPGs with goofy/dark humor.

Persona 2 and 3 are on PSP, and P3P at least is quite good.

FF1 and FF2 were remade for PSP, and FF1 at least is a fantastic port. They went all-out on the graphics and music to make it feel more like a modern game.

I've only played the first Valkyria Chronicles for PS3, but if Valkyria Chronicles II is anything like it, it's probably a cool game.

Not sure what issues you're having with Jeanne D'Arc, but it was one of my favorite PSP games by far. The FFT port for PSP has some technical problems, in particular spell/summon animation lag.

Here's a good list of PSP RPGs: http://www.neogaf.com/forum/showthread.php?t=464557