I should probably start with citing my favorite in the series. For me, it would be VI. A balanced blend of the earlier titles' fantasy/medieval leanings, with a nod towards what the writers would do full-blast with VII and VIII, adding the more steampunk and technological elements, but not to the point of overblown cliches and presentation the series (and JRPGs in general are now deservedly known for) would become known for with VII and onwards.
For me, it has the most memorable moments in its story, the largest cast in the entire series (no less than 12 playable characters). It just came out at the right moment and on the right platform. It's not without flaws of course....the Esper/Magicite system is laughably easy to exploit, the second half of the game feels unfinished (save for the minimalist remainder of the main plot.....it has a ton of side-quests and optional battles with eight INSANELY DIFFICULT dragons.....they were hard for me anyways), and there are some loose ends in the story that never get resolved by the (easily longest on the SNES) huge 30+ minute ending cutscenes and staff roll.
In my feeble attempt to break the nostalgia glasses I often wear when these sort of topics come up, it's hard not for me to notice that the series has lost a bit of that charm it had.
In fairness, I do realize that if we had today's technology back in the early/mid 1990s (the era when FF IV, V and VI came out in the U.S.), the games would have probably been in the same presentation style as more recent offerings like X, XII and XIII. You all know what I mean....fully voiced cut-scenes, no 'world map' to speak of, a more literal translation with only minor filling-in to make the words flow more smoothly in a Westernized fashion.
Something about the older iterations of the franchise, however, have something the newer games (from VII onwards....my opinion, but especially starting with X) are lacking, when looking at the overall story and the characters that help shape the plot. When my mind is allowed more free-will to imagine how the characters will look and sound, and (while in general, the overall story's start and end point can be cliche often times), not knowing which way the plot will twist or turn next, I find myself ore engrossed with a game in general.
Those aspects are what truly drew me into the older titles, especially VI. I was already 17 going on 18 at the time I played and beat that one, so I was possibly outside the target demo. I was still able to appreciate it, and probably still would today.
I think the other aspect is that, while FF has proven more popular Stateside than in its native Japan (even though it also does gangbusters over there.....nowhere close to what Dragon Quest does though), the other draw is that the story and characters were purely the creation of minds with East Asian ideals and the like. No one in the Western form of storytelling (unless I was missing some very niche storywriters at the time) was doing things of this sort. It had that air of exploring new un-explored territory.
The trouble I'm reading and seeing with the newer stuff is.....and I'm probably repeating others' sentiments. They're purposely fluffing up the story in ways that make it so convoluted and hard to follow. It's like taking a bunc of ideas, throwing them into a hat, shaking it up, then letting them fall onto a table in a random order that they refuse to change up, excise un-needed plot devices, and change up character models/sheets to bring some different things into the series. Older iterations did this well enough.....the first few FFs had the right mix of medieval fantasy, adding more touches of 'steampunk' with the middle of the series (VI, VII especially.....WHILE still keeping a focus on fantasy elements like use of magic spells and giving the characters still-relateable looks).
I could tell they were starting to run out of ideas with VII and onwards though. Starting to add, for one, more 'anime inspired' elements, tropes and character cliches. I understand by then, my generation was out, the new younger generation was in, but I knew they weren't getting the same experience I did. Or maybe they were and still are....just with different story elements and characters. I don't know. :)
The RPG genre (especially JRPGs), is really a barren wasteland though for anyone past their teenage years. Which is a shame.....there's lots of untapped potential. That's another topic for another day though. I personally feel Final Fantasy has run its course. They had some ideas going with XII, but seemed to fall back (or even regress further) with XIII. Didn't help that XIV was, to be blunt, an utter failure.
Japan's in the same economic mess (and has been for far longer than the U.S.) we are. They're just playing it safe, with few, if any, new ideas coming out there, with FF or with anything.
A shame really, as Final Fantasy really had something going for it during the 16 and 32-bit eras.