hey now... I'm starting a playthrough of the entire series... should I be playing the remakes instead?
My opinion:
FF1: As Corvo mentioned, it's basically a question of difficulty. The remakes got neutered to hell, making them pretty easy for the most part, but they look much better.
FF2: May as well stick with the remakes. The Famicom/(NES) version is an exercise in frustration and is riddled with bugs and other fun stuff. I'd recommend that version only if you're masochistic or have some good understanding of how its systems work.
FF3: Eh...it's a bit of a toss-up. I prefer the original Famicom version, but the remakes are pretty decent as well (not to mention much harder).
Both have their pros and cons, so you can't go totally wrong with the original or the remakes.
FFIV: No comment.
FFV: GBA version all the way. No contest here.
FFVI: Either SNES or GBA. The GBA version is probably more balanced and has extra stuff (and doesn't have the major game-breaking bugs like the SNES game), but the music quality suffers. But there's also a patch to solve that, so it's up to you on which you want to play.
You are forgetting the NES FFs. They are unplayable these days outside of the oldschool enthusiasts. Also, FF14's first launch was a bit of a disgrace.
The aforementioned FF2 gets the prize as worst, though, since it's one of the only FFs that really IS unbeatable if you don't know the system. At least 4 is balanced to prevent that.
Yeah, FF2 does a pretty horrible job of attempting to explain its systems.
There's a lot of weird and arbitrary stuff in the original that's pretty silly and luckily doesn't carry over to the remakes like the magic penalties from weapons/armor.
Even then, there's still stuff like Armor Weight that the game doesn't really mention.