I'm sure there are many threads already dedicated to this subject, I don't think that there's ever going to be a solid answer one way or another. In all honesty, I never finished 6. It wasn't bad, but I came into the series at 7 and couldn't go back. The controls were too clunky for me. I find whatever # I come into any series with is the furthest back in the series I can go. I don't like Burnout 2, the original Tekken or Twisted Metal, Soul Calibur, or Warcraft: Orcs and Humans.
Back to FF. Here's my list of the one's I've played in ascending order. Feel free to tell me how much I'm wrong.
8 (that game was terrible, unbalanced, and mind-numbing)
10 (just couldn't get into it)
5 (clever, but old)
6 (same excuse as 5)
12 (a lot of fun, made me forget about WoW for a month)
9 (wonderful story, graphics and score)
7 (the epitome of RPG's to date.