I don't think you could really say anything is "more" fantasy than something else when it comes a category that encompasses all of them.

If anything, I would say I-V were the "least" fantasy since they all used generic vanilla-flavored fantasy settings with little to no delving in to them. There wasn't much thought put into the locations and such, because at the end of the day, it was just a Dragon Quest knock-off.

FFVI is when you started really getting into specific crevices of the genre - steampunk, and the artistic representation complemented the idea well. Like VII, it also shared biopunk devices, storytelling about totalitarian organizations which biologically alter humans and other living organisms for control/power/profit.

VIII kind of went in the other direction, presenting a Utopia where nature and science coexisted, exemplified by the Shumis and Fisherman's Horizon.

IX might also be a little vanilla but it did include a wide array of locations, creatures, and legends.

I voted for X because it got so far into its concept that almost everything in the game was symbolic of it. Spira and its culture of death permeated every fabric of their world.

XII also was pretty in-depth with its middle eastern/east mediterranean features.

But if you want to add up which ones have the most amount of swords/magic spells/dragons/princesses, that'd be another thing.