I torn between Dragon and Phoenix, but personally I don't like the idea of having my mouth turned into a beak and no hands, so I choose seiryu

Yamata No Orochi has the same potential too, it just seems weak and not looking good because zoan follow each user facial characteristic, imagine if zoro eat it and do 8-sword style, probably zoro transformation form minus the hand idk if this zoan has limbs or not

Phoenix has a cool design .

I'm gonna have to say, Kyuubi No Kitsune.