Whats the hardest Final Fantasy?

Playing VII Rebirth on the hardest difficulty and really enjoying the challenge. Only played 6 and 7 Remake besides this, but I remember 7 having some good difficulty, and 6 being fun but not exactly 'difficult' outside of some select encounters (the tower?) and grinding. But thats more frustrating than difficult Id say

So which FF games are known for being the most difficult? Really curious as I do like my games to lean a little more on the challenging side personally. Tactics maybe?