I'm a little disappointed that Zygarde's HP was the only stat that got a significant boost in its Complete form, but holy crap, this thing is bulky. In fact, "bulky" doesn't even begin to describe how amazing 216/121/95 bulk is. Assuming this thing gets to stay in OU (if even for a little while), I think it has tons of defensive potential. The massive scale of its HP makes picking EV spreads a little weird, but if my math is correct, 172 HP / 120 Def / 216 SpD with a Careful nature gives it the best overall bulk. Here's just a couple of calculations to demonstrate how absurdly bulky it is:
252 Atk Life Orb Terrakion Close Combat vs. 172 HP / 120 Def Zygarde: 195-230 (31.6 - 37.3%) -- guaranteed 4HKO after Leftovers recovery
252 SpA Choice Specs Keldeo Hydro Pump vs. 172 HP / 216+ SpD Zygarde: 205-243 (33.2 - 39.4%) -- 16.3% chance to 3HKO after Leftovers recovery
Even then, you're probably going to want to focus more on one side of the defensive spectrum rather than split up the EVs like that. Let's say you run a fully specially defensive Coil set. With that much special bulk, Specs Keldeo no longer can 3HKO with Hydro Pump or 2HKO with Icy Wind, LO Latios has a pretty good chance to miss the 2HKO with Draco Meteor, Protean Greninja can't OHKO with its STAB Ice Beam, and LO Thundurus needs Nasty Plot to have a shot at 2HKOing you with HP Ice. On the physical side (after a Coil boost), CB Terrakion only has a tiny chance to 4HKO with Close Combat, Jolly Charizard X's +1 Dragon Claw will almost never 2HKO, Mamoswine's Icicle Crash comes short of a OHKO even with an Adamant nature and a Choice Band, and Mega Medicham's High Jump Kick does about 31% max after Leftovers recovery. So yeah, this thing is ridiculous. Coil / Rest / Sleep Talk / Thousand Arrows would be terrifying for a lot of teams to face.