Most DEFINITELY Final Fantasy X. It's like ten bucks and it'll keep you busy for a long while. It did for me, anyway. I'm STILL playing it. Probably my favorite FF as far as story goes.
My opinion, though, Final Fantasy X-2 is worth the cash, but doesn't live up to the first one. ...Final Fantasy XII was a big fat waste of money. Most definitely my least favorite FF to date, but different strokes and all that. Who knows, you may love it.
Hmm...I'd also go with Tales of the Abyss and Tales of Legendia (to a lesser extent; it's nowhere near as good).
Definitely all three Xenosagas. I own them all, but have yet to finish the second one. The first one is excellent. The battle system is perhaps a little too slow, but at least it's fun. The second one kinda sucks. The story is still intriguing, but the battles are devoid of any fun at all. I hear the third one mixes the fast paced battle system of two with the fun of the first one, all the while keeping the story interesting, so...go for all of those.
Seeing as I've never played any of the Suikoden games, aside from the first one on PSone (which is fantastic), I can't really recommend them. I've heard that the forth one sucks and the fifth one rocks, but I dunno.
Also, if you're really desperate and/or into the Final Fantasy VII storyline, I guess check out Dirge of Cerberus. I'm a big FFVII fan and I could barely get through it (and by it, I mean the horrible gameplay), but the story is neat, as it expands on the world after the events of FFVII and Vincent's (along witht the other characters') backstory.