Spin-Offs, Sequels and Prequels are part of one story so they all have the same ranking for me.

I. Compilation of Final Fantasy VII
II. Fabula Nova Crystallis: Final Fantasy XIII Trinity
III. Final Fantasy IX
IV. Final Fantasy X Saga
V. Final Fantasy VI
VI. Final Fantasy VIII
VII. Final Fantasy IV Saga
VIII. Final Fantasy V Saga
IX. Ivalice Alliance: Final Fantasy XII Saga
X. Final Fantasy III
XI. Final Fantasy II
XII. Final Fantasy