Well, they have remade FF 1 and 2 on the PS1, I own those 2 on PS1, and I think that they were interestingly update. They were VERY difficult on the NES, I won't lie. The first 2 FF's demand a great deal of patience - the majority of the time will not be spent developing the story, but instead your stats, as the enemies get pretty strong pretty fast. However, the remake on PS1 does feature a difficulty setting, so if you really don't have the patience to keep up with the enemies, they can be toned down, though I much preferred the challenge, myself.
FF3 is only available on the NES...and I really haven't seen it in America, not even remade. However, like the first 2, it is a pretty solid RPG...but was quite limited by the NES...so the story almsot seemed a little....vague.
FF4 has to be right up there for me with FF6. FF4 had an amazing story, well-developed characters, and is quite challenging. I really liked the gameplay, particularly the penalty for running away from battle >=) You can get FF4 ported to PS1 through a game called Final Fantasy Chronicles, which also comes bundled with the best RPG ever created - Chrono Trigger.
FF5, as mentioned, is interesting. It uses a reasonably intricate job system, leaving a lot of combinations for your party....heck, if you were so inclined, you could have a group of monks everywhere you go, taking out the big guys with meditation and remarkably damaging melee blows. I think the story was ok in this one, but overall, I preferred FF4 over this one. You can get this one ported to PS1 in the form of Final Fantasy Anthology.
FF6 is phenominal. In fact, if it weren't for FF4, this would be the only FF that stands in 1st place for me. It has an AMAZING soundtrack, extremely well developed characters, a first-rate story, complete with a crazed villain who will stop at NOTHING to get you out of his way....and recurring, rather funny villains that hate you eternally for having beaten them once or twice (think Ultros, gents

). The special abilities the characters get are really neat, particularly Sabin, if/when you see him, if/when you get FF6....it's just VERY well done, a very big recommendation on that. It is also available, packaged with FF5 through Final Fantasy Anthology.
And...yea...that's my bit ^_^