About Charizard-Y, I honestly think it should
stay S.
In terms of wallbreaking, nothing really even comes close, except maybe garchomp-mega. He's weak to rocks, has a sub-par speed, and has a bad defense stat, but honestly, that's really where the cons end. Let's talk about what he does do good.
His special defense stat, on the other hand, is actually quite respectable. 115 lets it just shrug off anything defensive rotom-w even does! Sun lowers the damage taken from water attacks, effectively negating one of his weaknesses altogether, and he can roost to keep himself healthy, which, with all the switches he forces, is not hard at all. You could honestly go so far as to forego hazard removal altogether and probably keep yourself fairy healthy, simply from the fact that defog removes both hazards, and you can literally just force your own hazards on them and wait for them to remove yours too. Fire/Flying on the other hand is actually not that bad of a defensive typing, if not for rocks, it actually has some handy resistances in fighting, fire, and immunity to ground, all being very common types.
His special attack stat, oh boy, 159 base doesn't play games. The raw power of sun boosted STAB fire blast from 159 is often too much for even pokemon that resist it. Fire/Grass gets you solid coverage, especially when both of those moves are well over 100 base power. It threatens a 2HKO on nearly every special wall out there not a pink blob, and anything uninvested that doesn't resist it is totally dead. The very fact that the only true counter to this thing is the blobs is just a testament to his power and an even better reason he should stay locked up in S.
Ok, for people who say heatran is a counter to this thing, let me tell you this.
Every charizard-y, without exception, carries something to hit heatran.
Be it Focus Blast (unreliable but beats tyranitar), Earthquake (I like this), or even Hidden Power Ground (I've seen it a lot), Heatran is a predictable switch just begging to be nailed by one of these.
Remember, he can even carry dragon pulse to nail those guys for some damage.
So to sum up, it basically has 2 counters that come in several times throughout the match to deal with anything he can take.
Definitely let this guy
stay S.
To those who say mega pinsir is strong and all, oh boy it certainly is, I mean, look at these calcs.
Mega Pinsir stays S: It doesn't matter if you're predictable when you wreck everything's shit (see Mega Kangaskhan). Mega Pinsir's Aerilate Return is absurdly powerful, and EQ takes care of most resists. As for those that resist both:
+2 252 Atk Aerilate Mega Pinsir Return vs. 252 HP / 232+ Def Skarmory: 142-168 (42.5 - 50.2%) -- 39.5% chance to 2HKO after Stealth Rock and Leftovers recovery
+2 252 Atk Aerilate Mega Pinsir Return vs. 252 HP / 252+ Def Zapdos: 198-233 (51.5 - 60.6%) -- guaranteed 2HKO after Stealth Rock and Leftovers recovery
+2 252 Atk Mold Breaker Pinsir Earthquake vs. 252 HP / 252+ Def Rotom-W: 292-344 (96 - 113.1%) -- guaranteed OHKO after Stealth Rock
With a bit of team support and/or prior damage, Mega Pinsir can break through them. Unlike Talonflame, Mega Pinsir is a late game sweeper that doesn't require constant switching out, so the 4X SR weakness doesn't matter as much. It's much easier to remove Pinsir's checks/counters before it sweeps, so it deserves the S-ranking. Yeah, it's not the most elaborate argument, but there isn't much to say.
I don't have much to say on the others. I just felt like addressing these two Pokes in my own words. I also support Mega Venusaur staying S and Garchomp/Mega Garchomp staying A+ in particular, though I'll just let the other arguments in favor of this speak for me.
However, a "bit" of team support and/or prior damage is a sad understatement. You have to realize that this thing literally needs rocks off more than friggin talonflame, atleast he HAS recovery and a good typing. Bug/Flying is utter trash, and honestly, how are you even going to get this boost?
How?
You can't take a hit without being totally crippled and you can't come in multiple times.
Let's just see what you even force out.
Aegislash: nope, he can take an unboosted eq just fine. Shadow balls back for big damage
Charizard-Y: You force it out, but honestly, I'd rather just stay in and risk it.
Charizard-X: 252 Atk Aerilate Mega Pinsir Return vs. 4 HP / 0 Def Mega Charizard X: 228-268 (76.5 - 89.9%) -- guaranteed 2HKO
I'd risk it and stay in
Mega Venusaur: You force it out
Deoxys-S: You lose to psycho boost, can only beat it boosted
Garchomp: Can live and Stone Edge back.
Kyu-B: Lives even after rocks and Ice beams back
Landorus: Sorta forced out.
Landorus-T: Why are you setting up on this
Heatran: You can't risk him having balls of steel and staying in. But you usually force him out.
Manaphy: You die to Ice Beam
Rotom-W: You get burned lol
Talonflame: You just die.
Thundurus: You just die.
Azumarill: You take big damage.
Bisharp: Forced out, mostly.
Conkeldurr: Forced out.
Deoxys-D: You might get T-waved.
Dragonite: Die to fire punch, live with multiscale intact
Excadrill: Forced out.
Gengar: You take big damage.
Greninja: You die.
Gyarados: Intimidate cushions it.
Keldeo: It needs about 22% weakening to gaurantee KO, otherwise you take big damage.
Lati@s: You take big damage/die to draco.
Mamoswine: You die.
Mandibuzz: Foul Play kills you if you try to set up.
Mawile: You take big damage.
Scizor: You sorta win, but you take big damage in the process
Terrakion: You die
Tyranitar: Max Hp lives eq easily, you die.
This might not look like a really impressive list, but there are three other things to take into account:
1) Rocks damage. Pokemon will try to finish you off more often if you're just at 50%
2) Idiots (or just realllly ballsy prediction). Sometimes people just keep their Conk at full health in on your Pinsir and just Ice Punch as you try to set up instead of switching. Idek :/
3) If this was a list of if you could take attacks from it, it would be a lot more straight forward.