6=7=8=10=12>5>4>1>3>2
Basically a big copout of the decision, but whatever. Never played 9, 11, 14, and not including spinoffs because its like apples to oranges.
6 had the best story and characters, 7 was the most well wounded, 8 had Triple Triad and used the Sci-Fi setting the best, 10 had the best presentation, and 12 had by far the best world and battle system for me. If you could somehow combine all of these, I would never play any other games again. I fanatically love Final Fantasy. Yes, even Mystic Quest.