Hmm... I'd have to say 5, because the story, while weaker than 4 and 6, was still decent, but the hooker for me was the customization options for the characters. (ie, the job classes) This variety had me wanting to do several different things, some of course working better than others.
Right behind 5 is 6, for a sheer epic story that is mostly unrivaled even today (and definitely unrivaled by current S-E projects), and the way that the sprites were able to convey emotion so well. Of course, I could keep going, but... well, I know 5 isn't a popular one, but I don't think 6 really needs much defending...
-dunno001
-On a quest for the truly perfect game; I don't think it exists...