For me, FFIV is the title that really changed the franchise. FFII brought story and character but IV was the game that made something of it by making the emotional conflict and turmoil of the characters be front and center as a driving element of the plot. This in turn became the basis for the rest of the series because while FFV tried to go back to the plot of convenience, FFVI and VII took this concept of the character-driven story to become the element the series would be known for.

Even though the plot has not aged well, people are still being drawn in by the characters which is a pretty nice testament to how Square got so much right.