As usual my choices are purely off design.
Out of the Legendaries, I think I like them both the same, but in different ways. I like Xerneas because it looks so graceful (I like the colourful antlers), and Yveltal looks cool as well, and it has good typing. Not keen on Zygarde or any of its Formes, though. Just looks like green and black gloop, like something from Ben 10.
But since the Mythical ones are included as well, my vote definitely goes to Diancie, because I think it's so pretty, especially its Mega form.
I like how Hoopa's Confined design looks silly and cute, but its Unbound form doesn't sit right with me at all. The elf-like feet and floating hands look out of place. Volcanion just looks strange.