all right, calm down dude. when I said his ability to stand against Azumarill proved his viability, I was referring to what I believe is Zygarde's best attribute, being able to take out what should be his counters. He can beat offensive dragons fairly reliably, he can beat defensive fairies most of the time, and he can hold his own with other threats like azumarill. How can anybody say this guy isn't OU viable? (that was directed at people in the gen 6 OU speculation thread claiming Zygarde is strictly UU, not anyone on this thread. PLEASE don't take that comment way overboard like y'all have on all my other loose comments)
Anyway, Zygarde's OU viablility is truly manifest in his access to powerful boosting moves (DD and coil), adequate coverage (ground+rock), priority move (extremespeed), his unique stat distribution that lets him not only set up easily on common pokemon, but defeat pokemon that theoretically check him, and his access to interesting moves like glare and haze.
Reasons he might not make OU are that ice beam and HP ice are common and dangerous, and his unique playing style will mean he might not get adequate usage.