My favorite RPG is Pokemon Platinum

18th out of all RPGs ever? Pretty sure that would still be top 1%, so not really that low

Oh and... why don't you like FF9?

It seems you havent played enough RPGs!

I didn't consider it particularly hard. I wouldn't even say it's the hardest among the Shin Megami Tensei series. Shin Megami Tensei Nocturne was significantly more difficult imo.
If you check this topic here where someone asks about difficult rpgs, most people mention Nocturne, and no one says Persona 3. In fact someone specifically says that Persona 3 is "not hard". https://www.giantbomb.com/forums/general-discussion-30/great-jrpgs-that-are-also-hard-difficult-challengi-1758427/?page=2

By story bosses you mean the ones that appear every full moon? That's interesting. I suppose they could be troublesome depending on how you've paced your progression in Tartarus.
But from my experience the story bosses were fairly simple, while the Tartarus bosses could be more tricky. The reason for that I assumed was because you have to fight story bosses every full moon, and the developers didn't want players to get stuck in the game. While for Tartarus, you could tackle it at a very slow pace if you wanted to. (You don't have to reach the top until the final day).
Some Tartarus bosses were balanced in ways where they were given a stronger offense than normal, but were given an unconventional weakness as well to make up for it. For example, you could defeat the three knights by charming them and have them attack each other while you defend. Or another Tartarus boss that only used physical attacks, you could defeat by equipping a Persona that nullifies physical attacks. (This is similar to how some Shin Megami Tensei Nocturne story bosses worked) There were no unconventional methods like that for story bosses in P3. It was basically, does it have an elemental weakness or not? And buff your party and debuff the boss.

I googled this to see if that was the general consensus or not, and found this topic where someone posed the question, if Tartarus bosses are harder than Full moon bosses, and everyone seemed to agree: https://www.gamefaqs.com/boards/971508-shin-megami-tensei-persona-3-portable/56124623

Was Persona 3 your first Shin Megami Tensei game? And did you buff and debuff? Because buffs and debuffs were more important in P3 than many other rpgs, and that can throw some people off. Or perhaps you were underleveled.

Elizabeth was a New Game+ super boss, so she was naturally tougher than the rest.
The hardest super boss I've fought was probably Seraphic Radiance in Shadow Hearts. I was no doubt underleveled, but I still fought it in what turned out to be a very long and difficult fight. Essentially I had to heal three times and attack once, or else I would die. And not only that, but I had to do a critical heal every time. In Shadow Hearts, the way to do a critical attack or critical heal is to time your button press to overlap with the small red area on the Judgement Ring.



For healing there is only one single strike section (instead of the five you see above for comnbo attacks), and I had to hit the red area each time. That's because my standard healing power was not enough to keep us alive, even though we spent three consecutive rounds healing for every turn we would attack. If I missed the red area even once, I would not heal at all, and would be dead.
This went on for over an hour, so that was a very long time to not be allowed to make a single little mistake.

That is why that was probably the most difficult boss fight I've ever been through. Of course if I had leveled up more it wouldn't have been quite as difficult as I made it.

Speaking of which, it's too bad Shadow Hearts didn't make the list. The first two are in my personal Top 5 or Top 10.