All the SNES FFs have been remade for PS1 and are very good (4-6)
12 I didn't like at all. It's the only FF I dropped out of boredom. Ever since Square became Square-Enix, they've changed Final Fantasy; the games are a diferent genre of RPGs, they are no longer Classic RPGs. Maybe you can enjoy that type of game, but I didn't... it's the only FF I actually ever regreted buying.
Oh, and then there's also FFX-2... if you are familiar with the FFX universe it's ok to play it, but it is far from being nearly as good as FFX. The plot and cast of characters suck, but the gameplay can be entertaining. If you want to explore the FFX universe further without setting your expectations high, then it's ok to purchase this game, you should be able to end up enjoying it.