If you like FFX, play FFX-2.. It was great in many ways (more cheerfull, enjoyable battlesystem, lots of sidequests, 5 endings etc. etc.).
Since you have a psp I guess you could try the original and FFII aswell, although I'd prob download 7 & 8 on your psp before playing those.
Well basicly they're all good, the only one I didn't like of those I've played was FFXI but that's an MMORPG, not quite the same.. ;)