Ehh I liked FF2, (real 2, not 4... I love 4 also, but I am talking about 2, as you are lol). It had a more developed plot, it had set characters, personalities and roles. It implemented keywords and special NPCs to advance plot. It was the first FF to introduce Cid!
The battle system is what people contently blast - the idea of the battle system is quite cool - you get better at what you do and what happens to you... your tank that hits things and gets hit a lot gets stronger at attacking physically and getting hit = more hp...same for magic etc. etc. If you used the system naturally and progressed there were a few typical FF "exp" walls to get past- people don't like is that you can gimp the system simply by backtracking to a earlier area with weak enemies and have your party beat on each other, nuke each other, heal each other to excess and jump endlessly in power- all you gatta do is grind... grind vs yourself, or grind vs NPCs...its still the same level/exp wall grind.
As I have said before FF games tend to flow in themes - as well as try and innovate the battle system. I see now that FF games have ALWAYS changed up and attempted to push and innovate battle systems etc. however - the US did not GET any of these (2,3,5...) and we only saw games that had fairly similar with understandable advancements, however mostly inline with each other 1,4,6 and 7... So me myself I tend to like the "normal" or "old school" battle systems and hate when their innovation in trying something new to me makes enjoying the game more difficult. (looking at you 8)
As for common plot idea- FF2 and 4 are very similar, as are 3 and 5. In 2 and 4 its a big empire with the might of airships going in an subjugating the world for power etc. they both hold a plot twist about a party member being bad or not...or bad... etc.
3 and 5 like 2 and 4 are similar in function (job classes) and plot - with this odd outside world misbalance threating the entire planet, lots of very interesting small and contained kingdoms etc.
8,9 and 10 again all are basically love stories- its like square gets stuck on something and cant give it up until they feel they have done it.
6, well 6 is an enigma and is seen as one of the best for many reasons - it innovates the battle system with something that is different (espers) yet- each person has their own "class" via not only personality but plot etc. While we started with a fairly strong focus on Terra, it later totally focuses on other members and focuses heavily on Celes - giving her some of the best moments of the game (Opera and post world destruction depression and attempted suicide, perfectly linked back to the opera house scenes... our hero's are real people, they are not perfect.)
The plot also, to many is a perfect piece together of classic and push to new items - the empire, the success of the enemy beyond the standard village burning.. also this is where they started putting more personal "love" stories into the plot beyond the price and princess happy typical high fantasy love plots.
To me the modern games just haven't done it for me... I am sure I am missing some great stories and great characters.. maybe I will give 15 a good ol "lets go on a bro trip in our car and kill dino's" try on my xbone Netflix and hulu streamer :-D