Severely overhated, in my opinion (taking later releases into account that is, the NES/Famicom version still sucks). I am one of FFII's biggest defenders.
The gameplay does suck for sure, however, I don't think it's quite as bad as people make it out. Again the Famicom version is just kind of bad but other versions are whatever.
The story, while absolutely nothing amazing, has a lot of ambition behind it and is ahead of its time in every sense of the word. Not very complex, but it is legitimately trying to convey some character work and themes here and it's really cool to see that in what would later become a very story focused series.
Great music too. Rebel Army my beloved.

The NES version of FFIII is significantly more polished than both of its predecessors. It's like they flipped a switch to instantly fix every problem those versions had. The problem is the fucking balancing. You have to grind after nearly every major story beat and it's so tiring. Definitely play the DS or Pixel Remaster versions if you want to experience III for this reason alone.
The job system in general is a bit disappointing. Advertises itself as having so much freedom and then a decent amount of the game practically forces you into an all mage party, or forces you to have dragoons, etc etc etc. It's not egregious, but FFV proved how far you can really take a job system. The gameplay in III does have its moments though, and it was obviously extremely important, starting so many iconic jobs and abilities from FF as a whole. I love the world and the general vibes, and the music wraps everything up in an enjoyable little package.

IV is definitely the hardest FF for me to rank. On one hand, it's iconic. On the other hand, I don't entirely feel it's that good...?
I feel in my heart that if there was one Final Fantasy I would pick as my least favorite, it's this one, however I do enjoy it more than I and III, so I'm not sure at the end of the day.
The gameplay is underrated in my opinion. The lack of freedom only allows the game to create tailored challenges for your party and can be decently tricky at times. It's on the right track.
The story is just fucking dumb. It hurts because the first third, everything up through the paladin scene, is genuinely fantastic stuff. And then it just drops the ball so hard. Cartoonish twists, hilarious fake deaths (and a shit ton of them), thematic developments that feel like non-sequiturs. I'm curious why I don't see more people talk about this stuff and I feel like the only person who doesn't consider IV to be the first great FF like most others. Still, that OST bangs and I do enjoy my time.

(Replay) The public consensus turns a blind eye to this game, and I truly can't fathom why. This game is the culmination of all Final Fantasy up to that point.
Absolutely peak gameplay, the best in the series. It's tough as nails, rewarding, complex, and outright requires you to plan and get as creative as you can. You never have to grind at any point, you just need to figure out what you could change about your strategy and then carry it out. If you primarily care about gameplay, this is the #1 must-play Final Fantasy for you.
The story, often hated on for being comparatively very simple or even childish when it's right next to VI, VII, IX, etc, is very much still good in my opinion. Not great, and not without its flaws, but it's cute, the characters are extremely enjoyable, Exdeath is peak, and the graphical leap since IV allows for some extremely well-directed cutscenes with perfect animations.
The soundtrack is also amazing. It's Uematsu. I don't need to elaborate.
I firmly believe FFV to be the best in the series in every single aspect up to that point. It can't touch the following classic entries for story, but in every other area it absolutely measures up, and in gameplay, clears all of them.
Very happy I replayed as I had forgotten most of it and happy to say it has risen above VIII for me. The most underrated FF by a lot.

FFVI stands tall to this day as one of the best stories in gaming. The characters are beautifully written in service of the themes in a way that still hits 30 years later. This was probably the first video game to have this good of a story, and it's truly remarkable for its time and has aged wonderfully. It's powerful, extremely emotional, the presentation is great, the music is phenomenal. The gameplay is boring but I don't care about that in a game like this. Makes me cry at multiple points every time.

To say this game is special to me would be an understatement. Final Fantasy IX changed me. It literally changed my life and I would be a very different person without having played it. This game gave me direction, it gave me motivation, it gave me a reason to live. It taught me that life is beautiful and that leaving even just one positive memory in someone's mind means your life was worth living. This is my second favorite piece of media ever made and every day I feel like a better person because of it.