Not bothering with the sequels or spinoffs.

Tier 1: VI, VII, IV, V, IX
Tier 2: XI, X, VIII
Tier 3: XII, I, III
Tier 4: XIII, II

I probably could put VI and VII in a tier of their own. They really are fantastic. I could see my other ranks changing over time but nothing can really top those, or Chrono Trigger, my third favorite game ever.