Well, you have to tip your hat to FF1, its THE most important because had it sucked, or not been just right in its ability to keep people playing it over and over again, the series would never have continued.

IV is important, because it was the next one that was decided to bring to the US, which also brought VI and VII.

VII is important, it was an early PS game which was probably one of the earliest+bestlooking RPGs people had ever seen, and it attracted probably the most attention to the FF series than ever before (a new generation you could say)

Considering importance in the series, these 3 are prolly the most, not considering gameplay/storyline/whathaveyou, which is a totally different topic.


FFV: Last game to use crystals as a major plot device
FFIX. Which is expected considering FFIX was also a huge salute to the first generation final fantasies (job classes, crystal theme, 4-man parties, etc)