If you have $13 to burn, Chaos Rings all the way. It's the most polished game by SE, and it has the most value in terms of game length, if you consider the parallel story lines as unique play. Also, it's graphics are very good. The original ffs are really only for those who were played the game back in the day, for nostalgia. Chaos Rings is what you're looking for. Of course, if you want to wait for Final Fantasy Tactics, that's more of a strategic game. If you like strategy, wait for that.
Vanguard Storm seems to me a very good game at a much lower cost. But I only tried the demo and it is still in my wish list. Chaos Ring isn't yet in my wish list because I couldn't try any demo but in long term despite the price I'll probably add it in my wish list and will probably end to buy it.
Yup defiantly Chaos Rings. Song Summoner does have quite a bit of strategic game play but the fact that you need to listen to your own music a lot to get strong people is annoying.
Go for Chaos Rings. Don't even bother with FF1 unless you're a big RPGer. If you want a Final Fantasy wait for FF7, 8, 9, or Tactics to hit the iphone (someday)