Satisfying presentation and story importance tend to slightly trump challenge for me when it comes to final bosses, especially if that boss has been a great villain, so I would push Ardyn (my favorite FF villain in a long time) up to #7, Vayne to #4 and Kefka to #1. I also deeply dislike Giant Space Fleas from Nowhere, so Cloud of Darkness joins Necron at the bottom.

Otherwise, solid picks. *approving nod*