I really can't choose between VII, VIII and IX. Like I know VIII is the most flawed out of these three, but I still love it to death. And VII is probably the most rounded one as well as the most iconic.
But IX was my entry point when I was only 7 and is probably my favourite game of all time. VII and VIII are incredibly close and equally nostalgic, though, and their vibe is closer to what I enjoy more these days, particularly VIII's setting and artstyle.
Had I played VI at an earlier age instead of my teens, I'd probably rank it among VII, VIII and IX, but I still consider it one of the best for sure (probably the second or third best) and love the cast. XII just needed a bit more of a hook for me in the characters, but I still liked them a lot and the game is genuinely great.
X is a great game, but there's a lot I sadly don't like in it, even straight-up superficial shit like character designs. And VII Remake was astounding, but, for obvious reasons, lacks the journey aspect that the others have and there are some important narrative choices and execution that I genuinely dislike.
So I guess IX?