Zygarde C- > C
I'm gonna make this short since I have to go soon but ...
Honestly, Zygarde is a really underrated threat. It has a fantastic typing and very good defenses, allowing it to easily set up Coil or Dragon Dance. Even though it has lackluster Attack, its ability to easily set up a DD and survive most priority even after the damage it took while setting up is amazing. With Leftovers, it could possibly set up two Dragon Dances, and with Life Orb it can most likely sweep through with 1 DD. Zygarde also has Extreme Speed, dodging the Talonflame's BB and often (Esp. with rocks up) will pick the KO with ES. However, its bulk really is the biggest selling point, which I think warrants it at least a rank of neutral C.
Just to give you guys an example of its bulk:
+4 252+ Atk Technician Mega Scizor Bullet Punch vs. 16 HP / 0 Def Zygarde: 306-360 (84.7 - 99.7%) -- guaranteed 2HKO
If you guys think "oh scarf lando-t is so common it just stops" it actually is not stopped by Lando-t (Just forced into Outrage).
252 Atk Landorus-T Earthquake vs. 16 HP / 0 Def Zygarde: 151-178 (41.8 - 49.3%) -- guaranteed 3HKO
Landorus-T can't even 2HKO and if he came in on a DD, then 1-2 more DDs and you got the game in the bag.