Did you ever play 11? The combat system is strikingly similar, albeit different features (such as exclusion of chainspell mechanics.) Some of the code was most likely recycled for 12.
My personal favorites are 6 & 9, but I'm liking the Ivalice setting more as I get deeper into 12 (never got around to completing it,) the combination of magic, technology, and medieval europe is very interesting to me. I like the fact that they're developing games that use the same world, makes for a better detailed setting.
It is looking good and Versus XIII is definitely looking interesting, despite Nomura's obsession with "beautiful boys."