Best:
I - The nostalgic appeal and simplicity of it puts above the rest.
VI - Where the series peaked imo. Best game ever.
VII - Fantastic follow-up to VI, breaking new ground in 3D modeling while still keeping to the traditional elements.
XI - A completely biased pick because I played the smurf out of this game with my wife. A lot of great memories. Story and job system were also very solid.
I guess XIII-2 - Because I only played this and XIII and XIII had too many flaws, most of which (at least gameplay-wise) were fixed in XIII-2

Worst:
II - I just hate that battle system that requires you to beat up your characters to advance them.
V - Nothing against this game but it's up against some juggernauts! Definitely the best game on this 'worst' list.
VIII - The contrived love story and draw magic system bring down an otherwise fun game.
XII - Another solid game, I just liked the other two better. In XII I did tend to get lost in the side quests and then lost interest in continuing (for like 3 years).
XIII - A flawed game in so many ways. It had some high points and generally I enjoyed it but it just felt like a mess of parts pieced together.