I'd suggest you start with Star Ocean: The Second Story. The PSP version fixes a lots of things, changed the voice actors casting and added tons of voiced dialogues. It did some weird changes too, like removing "Earthquake" magic from the last boss to another one witch is still as deadly as "Earthquake".
Finally, the PSP version doesn't come with the awesome "We form in crystals" theme that runs during the final CG cinematic when the credits roll, and used a very, very bad J-Pop song instead. Bleah !
At least, you can still hear the original music in the Sound test once you complete the game.
For me, SO 2 is the best in the series, way better than 3, 4 and even 1 with the atrocious cave dungeon (do you remember the Moria ruins in Tales of Phantasia ? It's worse).