The Good: VI, IX, VII, IV, I
The Meh: V, III, IV After Years, X, X-2
The Bad: II, XIII, VIII, XII
Each grouping is ranked from best to worst. The Good and The Meh lists are mostly subjective in terms of ordering.
I think that most would agree, however, that the games in The Good generally do more things right than the games in The Meh.
I've ranked The Bad based on how many things I feel are utterly broken in each game:
XII: The Battle System, The Characters, The Plot, The Leveling System (The License Board).
This guy explains the problems with XII better than I can:
Part 1:
Part 2:
Part 3:
VIII: The Battle System, The Leveling System, Most of The Characters, The Plot.
XIII: The Linearity, The Leveling System, The Plot.
II: The Leveling System (Although this has been fixed somewhat in later releases, I originally played the fan translated NES version).
I've never played any of the MMOs (XI and XIV) or any of the XIII spinoffs (XIII-2 and Lightning Returns) . . .