I was never able to bring myself to finish it, but I kinda enjoyed 13 to a degree- this is likely because I've only played a few FFs, and I tend to look at them a little differently than fans that have been with the series since the onset... I guess. I also enjoyed 12.
The battles in 15 just look so organic, and everything looks really alive. That scene with the Garula(s) is sticking in my mind.