I want to say 2 words about this mon since im prob one of the players who wanted this suspect the most.

Like ABR alrdy said in the first post of this thread, this mon has a large viability of sets that can use and if by any chance (you dont even need to be that lucky) you have the right set at the right moment you can win the game just by playing 1 vs 6: some example are the games that Lyd used on his post.

One of the thing that makes zygarde broken in my opinion is the fact that it (can) win(s) even against mons that are in the opposing team just to check it, such as Tang which loses to Z-Dragon, gets stalled by SubProtect Toxic and can also loses to SubGlare, Clef loses to Z-Steel, Z-Ground, Weakness Policy, Choice Band and has good odds to lose against SubGlare, i would not put Lando-T even as a check since it has 3 switchs to arrows from the cb set and others just destroy it, even Bulk Up Bulu aint a decent check to Zygarde since as all the other fairies loses to Z-Steel plus loses to SubToxic and cant even check SubGlare since it either gets haxed by an arrows crit (which is around 50%) or pp stalled even by being parad (you can try out yourself). Everytime a new Check/Counter for Zyg comes out a new set that beats the new Check/Counter borns as well (Dragon Tail for Curse M-Zor is an example, so is Rest for Toxic users and so are Z-Dragon for Tang and Z-Steel for Fairy). This viability gives Zygarde the chance to be played in every playstyle (Stall as well) and never being a dead weight since is nearly impossible that this mon gets a match up that makes him not viable in even 1 single battle. If this is not enough (even though it is always by my opinion of course) it gets a good trapping move such thousand waves that if used in the right moment can literally gift a win or a mon at least and a great +2 priority with 80% base power that can force many of his revenge killers out. Talking about revenge killers there are not either many pokes that can revenge kill it since after a dragon dance the only mons able to outspeed it are ScarfGren (that needs to be out of Espeed range) and a full-Hp Bulky Lando (that needs to carry HP-Ice and to not face a Z-Zygarde or WPolicy) and Tapu-Lele (who goes into a tie in the best case and needs Zyg to be chipped before), other sets such Coil that dont boost speed dont make Zyg easier to revenge kill since it can live Leles moonblast (252 SpA Tapu Lele Moonblast vs. 192 HP / 0 SpD Zygarde: 324-384 (80 - 94.8%) -- guaranteed 2HKO after Leftovers recovery  by only using the 192hp(405) needed to not get sub broke by chansey), Perts Ice Punch (252+ Atk Swampert-Mega Ice Punch vs. 192 HP / 0 Def Zygarde: 340-404 (83.9 - 99.7%) -- guaranteed 2HKO after Leftovers recovery) Garchomps Outrage (252 Atk Garchomp Outrage vs. 192 HP / 0 Def Zygarde: 336-396 (82.9 - 97.7%) -- guaranteed 2HKO after Leftovers recovery) and many others you probably should already know.

So my thoughts are that this mon is too much for the Overused tier since it either wins or makes holes to let other members of the team have an easier sweep, just BAN this toxic and unbalanced mon for a better tier.