I disagree with the above quite a bit particularly on the issue of Zygarde, I think the mon is quite a bit overrated and I want to give my reasons why. Just to have a comparison point; while don is another mon I am known for liking, I do think it should be ranked higher than Zyg and I'll give some of my reasons why below:
To start off, Zyg is often considered by far to be the most "annoying" mon in the tier, and it's pretty easy to see why. It often relies on Glare to get things done, and losing to Zyg just because you happened to get parad when he subbed is a pretty lame way to lose. It can fish for subs a lot of times thanks to Power Construct. In addition, there is the "demon" zyg set consisting of waves,scale shot, coil and rest; this is another annoying set because it can often just autowin some games. You can easily feel robbed because you just happened to switch something wrong in on the demon set and autolost from there. Because of all of this I think the opinion of Zyg in general is often overblown simply because of how damn annoying the mon is. However, I don't actually think it makes the mon insanely good like others think.
For one, having a cleric greatly weakens Zygarde's ability to actually make progress. I think this point is particularly salient because many, many players skip out on having a cleric on their balance teams. It's kinda your fault if you decide to run Grass Knot for the 1 in a billion Tyranitar your Calyrex will run into and thus lose to Zygarde, when you could have ran Aroma which has utility against almost every team in the meta. Xerneas is another big cleric, and it in particular has a fantastic matchup against Zygarde, and no, I'm not just talking about the rare Phys Def Xern, as Aroma fits fantastically on Geo sets as well. Blissey can also easily fit Aromatherapy and run Ice Beam to force Zyg out. You can even run Heal Bell as a utility option on certain Gothitelle sets. And while Arrows does get neutral damage on the vast majority of the tier, it's really not a strong attack even when boosted; thus, it's not like you typically have to aroma many times before Zyg is no longer able to threaten you.
The second point I want to make is about defensive utility. I think this is another point where Zygarde's prowess gets overblown, as Zygarde seems like a mon that should have tremendous defensive utility given Power Construct and it's HP stat post-transforming. However, when you actually analyze it, it simply does not perform as well as you'd imagine. For example, let's analyze the Groudon matchup. Let's take the best case scenario and use the very defensive Coil Glare Rest set. This set seems great against Groudon, being able to paralyze and coil against it. The thing is that, LO Groudon has a huge chance to just OHKO Zyg straight-up with Precipice Blades:
+2 252+ Atk Life Orb Groudon Precipice Blades vs. 204 HP / 248+ Def Zygarde: 394-464 (96.5 - 113.7%) -- 75% chance to OHKO.
Another very common set in the Lum Berry set, which will just eat away a Glare for free and proceed to get to +4 and kill you the next turn. If Groudon happens to have a Polish up already, even if you Glare Groudon, Groudon will still be faster. The best chance Zyg typically has in this scenario is to hope for Pblades misses. The offensive Zyg sets just lose outright. It's not like this is an uncommon scenario either,
you can see this interaction with my game with Solo from MW.
Let's take another one of the most common breakers, Zekrom. It will blast through any offensive zyg set, and defensive sets will typically lose to LO sets. This is not to mention Draco Meteor which will batter any Zyg and is becoming an increasingly viable option as a way to hit Tangrowth.
I would say Zyg's best role is its ability to adequately check Marshadow, but this is really only limited to the defensive set, as the offensive sets which have no recovery besides lefties and construct can be broken:
+1 252+ Atk Life Orb Marshadow Close Combat vs. 16 HP / 36+ Def Zygarde: 305-360 (84.4 - 99.7%) -- guaranteed 2HKO after Leftovers recovery
This is, of course, not to mention the other top offensive threats such as Zacian, Xerneas, Kyogre, Eternatus, and Calyrex, all of which can break through quite easily. Offensive Dusk Mane sets, while they don't love paralysis, can threaten to smash through Zygarde if Zygarde ever tries to damage it due to Weakness Policy. Even some more niche stuff like Caly-G, Darm, and Urshifu can put in work against it, opening up your defensive cores.
I am omitting the haze set which is quite frankly not optimal and seems like a worse Luna/Lugia.
As for the demon Zyg set, in its purest form it's a match-up catch. As I said before, the set is heavily dependent on what happens to come in on it. What if your opponent happens to go to their Whirlwind Ho-Oh on your waves? Now you've not only made no progress, but have also revealed what the set is and will likely not be able to do much with it for the rest of the game.
All of this typically leaves me big questions as to why I would run Zygarde on my teams over, say, Groudon. For one, don with it's lack of Dragon-typing actually checks Zekrom very well, and you will not be caught as hard from random Ice-type attacks. You can even have some defensive utility against Zacian. In fact, I would say Don just straight-up has more defensive utility than the offensive Zygarde sets. But most importantly, it'll actually be able to break through against balance teams with brute force instead of relying on status and luck, especially if the opponent has a solid cleric. Offensive Zyg sets suffer from a lack of defensive utility and being unable to break through solid defensive cores with clerics, not to mention having to be very wary of status due to lacking Rest, and while defensive Zyg sets do have more defensive utility, it still struggles to deal with the major offensive threats and breakers of the tier, and it has a very difficult time breaking through increasingly common defensive mons such as Tangrowth and Lunala.
To support my argument even further, I'd like to point out how Zygarde has had win rates of around 40% for quite a long time, which is honestly decently low considering how often the mon is used. Never have I seen a mon that is so hyped and simply just not having the results to back it up.
Round 3 of SSNL had Zygarde at a 36% (!!) win rate, meanwhile Groudon was nearing 65% win rate. Round 4 wasn't kind to it, either. As I said before, I think the annoyance factor of Zygarde pumps up its image in people’s head. I think a major part of Zyg’s low win% is how Zygarde has absolutely no role on HO teams, which have been becoming more and more common as of late. I think S rank represents mons who you would have to seriously consider running on every team no matter the team playstyle/archetype and I don't think Zygarde fulfills this role at all. I believe Zyg deserves A rank and while I don't expect this opinion to be common or agreed upon by the majority of players quite yet, I hope my post will help give you a differing perspective on an often hyped and hated-upon mon.
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Better Web setter can force stalemates with opposing webs due to Magic Coat, utility in Yawn is very underrated forcing free turns in your favor. Shuckle Webs also at the mercy of Taunt Gothitelle much easier to utilize Webs with Puff barring Scarf Tricks.
Down to A+ it's not better than the 3 others in S, the offensive variants are very good very strong but defensive sets are mostly Goth food barring Dragon Tail which exert no real offensive presence and at the mercy of other dangerous offensive threats. It's a shaky check to Kyogre as is with the inception of the Block / CM set and even then already shaky requiring some very good predictions to not get steam rolled. 
C- to UR: I dont see any reason to use this thing at this point of the gen. Toxic spikes? Eternatus, Toxapex or Naganadel. Bulky water type? Kyogre, Toxapex and even Gastrodon. Some what special defensive mon? Just use Ferrothron, Eternatus, Blissey or even Shedinja. And no, Rapid Spin is not a real reason to put this in C- 
C- to UR: After the DLC 1 i don think i see any utility for corvinight besides be a some what check to Groudon, Necrozma Dusk Mane and even Marshadow, but outside of that i think all of that mons can over pass this thing without to much effor. Also this is just a one trick pony, cant run some nice options on his sets like body press with iron defense if he needs to use roost and defog to have some what utility for the tier. Its heavily outclassed by Ho Oh and other physical walls like Buzzwole or even Tangrowth wich offers more utility for the team. 
C- to UR: I dont think this is the first option for some one when you think in a magic bounce usser... Cause Mega Diancie is not in the game anymore. Poor matchup against DM the main rock setter of the tier. 0 utility besides teleport (Wich as fragile as it is donst really work) To terrify from knock Off and the defensive utility he provides is close to 0. Legit use Espeon over this think, at least she can use Yawn to punish switchs and have mystical fire.
From Unranked to C-/C
From A+ to bottom S
From A- to A
), Blissey is able to do what it isn't able to in past gens: walling Choice Specs Kyogre. In addition to that, Blissey is a great utility Pokemon, with access to Stealth Rock, Aromatherapy, Teleport, Confide (allowing it to wall GeoXern with Rest or CM Kyogre), Thunder Wave or Healing Wish (for HO teams). Then, despite awful Attack and Special Attack stats, it's able to do decent damages with Seismic Toss. Overall, Blissey is a wall to the large majority of special attackers which can be found in Ubers, and isn't just a wall as it's useful for the team.
From C to C+/B-
to C+/B-
