I’m in the highly recommend IV and VI camp. Both have amazing music too. Worth it for the music alone.
I played them in this order:
VIII - love it, one of my favourite games ever
IX - love it, one of my favourite games ever
VI - love it, GBA version is definitely better than the PS1 version though, I assume the awful slowdown from the PS1 version is gone from modern versions
X - love it, played many times
VII - late to the party with this but loved it too
X-2 - only FF I ever played with a guide which I regret, but cool game
IV - loved it
V - it was ok
XII - loved the gameplay, played it to death
XIII - it was ok
VII Remake - loved it, one of my favourite games of the last gen
Think that’s it so far.