I'm pretty sure I have played nearly every FF game ever made in nearly the same year it was released (there are a few exceptions, XI, X-2, and XIII-2). That being said VI, IX, and XII stood out for me. VI is just a masterwork of storytelling and XII challenged the tactician in me to find interesting ways to alter my teams AI for any given encounter. IX was just such a fun nostalgia ride.

On the other side of the spectrum is FF XIII. I tried replaying that a few months ago and man....I didn't last an hour before I just gave up.