So I have actually been going through all of these while waiting on elden ring, currently on 5.
FF1 - Not a lot to say, it's easier than NES and looks nicer than the GBA/PS1. Would have been nice of it had the extra GBA content but still enjoyable with the QoL stuff like autobattle, sprint, and diagonal movement. It's still pretty amazing from a game design standpoint how they gated the areas/world so well 30ish years ago.
I prefer the PSP version to the PR version myself, because of the extra Dawn of Souls areas and the new secret dungeon at the end (so like, five bonus areas IIRC, and they're all good). Don't have a PSP anymore though. I guess I could emulate when needed. Thing is, the PSP versions of 1, 2, and 4 are all vastly superior to their PR versions specifically because of all the extra content they have. The PR versions of 3 and 5 are probably the best way to play them though. I don't even know if the PSP got those.
FF2 - this one is completely busted. I am not talking about the shitty leveling system everyone knows about already, which is still present. I am talking weapons and enemies will both proc status effects 100% of the time, even ignoring immunites. You can get wiped out in an ambush of mobs that proc stone/death before you can take action. The encounter rate is also ass. It ended up being an interesting game though as I essentially ran from every random battle and cheesed bosses by hitting them with swords that procd curse and sleep.
I've played through FF2 like 3 times I think and I always do all my powering up right at the beginning of the game. A couple hours of the characters training (by beating each other up) and I'm halfway to endgame strength before I even go to the second town. So I've never had any real problems getting through FF2. But yeah it's easily the worst of the mainline FFs.
FF4 - Cheated here and played the PSP version since I wanted to do the after years. This game still rules. The after years expansion took me about as long as FF1-4 combined. It is pretty grindy but if you like FF4 it does pretty well with the characters and adds Chrono Trigger like combo attacks called bands. The end dungeons are massive and have bosses from FF1-6 in them. None of that is in the pixel remaster which really sucks.
PSP also has Interlude, which is like an hour-long section of the game that no other version has and takes place after the finale of FF4.
The end dungeon of FF4 TAY is absolutely massive, maybe the biggest single dungeon in the whole series, IDK. It's like 30 floors and each floor is a full area, with like 20 bosses throughout the whole thing. I usually beat FF4's main game in about 9 hours, while TAY takes like 25-30 and half of that is the final dungeon. I found it super interesting with the idea that the endboss was the creator of all the enemies you've fought in other FFs. It was the first (only?) time they ever made an effort to connect the series together.
I remember Deathgaze (aka Doom Gaze) being the big threat in the last dungeon and taking me a bunch of tries to beat. Same goes for Atma Weapon and Shinryu. Then I got to the penultimate boss (blue-haired death lady and Dark Bahamut) and got one-shotted before I could even do anything by Meganuke. Turns out said penultimate fight is a big HP/level check. So I had to go grind...a lot. The girl I was dating at the time was already feeling iffy about me after a year since I was still a student and she wasn't, and me spending days working on TAY during a time when we were already on the rocks a bit was sort of the last straw for her so she just sorta left. I remember leaving the game running on like Depths B28 and going to talk to her, it not going well, and me going back to the level-grinding and feeling like a complete douchebag.
...aside from FF4 TAY getting ruined for me at the end, it was one of the best FF experiences I've had easily. The girl came back later too, though only for like another month before deciding she made the right choice the first time. Whatever.
Like I said before, FF3 and 5 are the two that I think the PR series has the best versions of (maybe 6 too? IDK) so I look forward to playing them the most whenever these get their console release.