I - Still good after 22 years. It's very simple and certainly not as deep as later titles. The best version is the PSP version, closely followed by the Wonderswan Colour...
II - Also very good, this is without doubt the hardest in the series. Adds a little more complexity to the story (not much) but is marred by the ridiculous levelling system and difficulty. Again, look for the PSP version.
III - Personally, I was never impressed with FFIII. The only official English verion (for the DS), whilst highly polished really spoiled the charm of the fan translated version that I had been played in the past.
IV - The first of the 'modern' Final Fantasys (IMO), IV gives an excellent story, a vast world memorable characters and much enjoyment
For me, again the DS version kind of spoils the charm a bit, but it's certainly well worth looking at. DS version has the best translation and is very shiny, but the GBA version is still well worth picking up. Avoid the PS1 version - poor translation and terrible, terrible loading times. Worth it for 'You Spoony Bard!' alone
V - Probably my favourite of the 'Nintendo' FFs. Again, a compelling story with fun memorable charaters (Gilgamesh anyone?
). A real rollercoaster tale of loss and sadness (mostly). Widely regarded to have the best music of the lot. Look for the GBA version, avoid the PS1 version for the same reasons as above (though slightly better translation in the PS1 version).
VI - Still regarded by many to be the best in the series (though this is contested with FFVII). My plus points are going to be very similar to IV & V here. A *huge* array of playable characters, with a decent range of Espers too. Contains probably my favourite Final Fantasy 'moment (**** di Mezzo Carattere). Sounding like a broken record here, but avoid the PS1 version, it's gash. Look for the GBA version, or import FFIII from the States with an import SNES.
VII - Needs no talking about. Play it, simples
VIII - Underated in most people's opinion. Most were a bit disappointed at the big change in style and presentation from FFVII, but most had never played an FF before VII. Big departure from previous titles anime styles, VIII has more realistically proportioned characters in a believable world. The draw magic system has many fans and many haters, IMO it's a good system, but a little easy to overpower your characters. Personally, I love this title. Yes, the characters are pretty weak for the most part, but the story moves along at a fair pace after a slow start, and for me is generally quite exciting 
Massive world with plenty of secrets too (which IMO is better than VII's world).
IX - Again, highly underated. Certainly seems to be easier (and shorter) than VII & VIII. Again, a highly enjoyable title with a welcome return to 4 character parties. Not as many side quests as the last few titles.
X - It took me a couple of attempts to play through X (mainly due to house moves). Overall I was very impressed in the end. Sadly, doesn't contain an overworld like all previous titles, which may disappoint. A decent story, levelling and combat system, marred by poor English voice acting. The Blitzball minigame (which is compulsory at points) won't be to everyone's taste. Some truly astonishing side bosses wait for you after finishing the story 
X-2 - Dear gods, avoid like the plague.
XII - Personally, I really didn't enjoy XII. Overall it feels like a real departure from the spirit of the previous games. Plays like a single player MMORPG. Many rate the game highly, but it's not for me. Also again has very poor voice acting.
*whew*
