Star Ocean 3 was the first in the series I played, and I love it. The battle system, the look of the game and the characters just clicked with me. And the soundtrack is just plain awesome. The story isn't necessarily a high point, but I've finished the game and that's not someting I can say about a lot of games.
I got a used copy of SO2 for the PS1 later on, and I almost finished that one. I got sick, so I had a lot of time then . It's a really fun game too. I should probably find the time to finish it sometime.