Entire series (Spinoffs/movies/OVAs included)
I - VI: Never played.
VII: very good. out of what I've played/watched this is second best.
VII-AC: Best.
VII-Last Order: Great... needs to be translated into English and expanded.
Other VII related spinoffs: Never played/watched.
VIII, IX: Never Played.
X: Only got a little bit through this one.... graphically better than VII, but didn't grab me.
X-2: Better than X in storyline department (IMHO). About 1/3-1/2 way done with this one.
XI: Don't have the funds to play.
XII: Won't be released until October
XIII and related Spinoffs: Still in developent.
Legend of the Crystals anime: seen bits and pieces of it... not enough to make an opinion.
Spirits Within: What got me started with the whole franchise. granted it's not great it is what has inspired me so I can never say it was the worst.
Unlimited (anime): ughh.......

I don't think I've forgotten anything...