Honestly, my ranking of most of these games is a product of when I played them and the impact they left on me.

Best to Worst:

FFVI
FFTactics
FFXII - Zodiac Age (More balanced than first version)
FFVIII
FFIV
FFIX
FFVII
FFX
FFV
FF
FFXIII
FFX-2

Have not played:

FFII
FFIII
FFXI
FFXIV
FFXV (Will probably give this a shot now that it's "finished")
FFXIII Sequels

Take care all.