Both III and V are on PS2. I would recommend V first -- the games are set in different kingdoms within the same huge world, another unique thing about the Suikoden series. It doesn't matter what order you play them in, but each game teaches you a little more about the overarching world.
V is great, III is great but quirky. I know a few people who love V and II but were annoyed by the combat system in III, because it's very different. If you end up loving V, then I'd definitely try III.
In V you play as a Prince in a Kingdom where only females can ascend the throne. Your future job is somewhere between messenger and general, but you're young and still training for it. Then everything starts going wrong.
In III you choose one of three characters. Eventually you play all three stories, and then they merge with each other in an interesting way.
A mercenary leading a gang full of younger mercenaries, male and female, who are extremely loyal but a little disgruntled by the jobs they've been getting.
A boy of a small tribe sent to the city on a peace mission. The boy has a pet Griffon he rides into battle... its awesome.
A female knight general from that city, sent to give the boy's village an ultimatum. She's one of the most powerful warriors in the game, and probably a little bit modeled on Cecil from FFIV.
Okay, now I'm going to bed.... I swear...
