I think that Mega Charizard Y should be moved down to A+. To begin, I would like to revisit the definition of an S-rank:
Reserved for Pokemon who can sweep or wall significant portions of the metagame with little support, and Pokemon who can support other Pokemon with very little opportunity cost ("free turns"). Also the home of Pokemon who can easily perform multiple roles effectively, increasing their versatility and unpredictability. If the Pokemon in this rank have any flaws, those flaws are thoroughly mitigated by their substantial strengths.
Zard Y cannot sweep a majority of the metagame. Or wall a majority of the metagame. I think we can all agree on that. Therefore, for it to be S-rank, it would need to either be incredibly versatile (it has ONE set) or be able to support other pokemon with very little opportunity cost (I wouldn't call being setup fodder for Zard X, Gyarados if it plays smart, and Mega Tyranitar, 3 of the best setup sweepers in the tier, all of which only need the one turn of setup you get by coming in on Zard Y, very little opportunity cost). In addition, Zard Y has tremendous 4MSS. Fire Blast/Flamethrower and Solarbeam are mandatory, then for the last two slots you have a choice between Roost, Dragon Pulse, Focus Blast, Earthquake, and Flare Blitz (lol). Without Roost, you get destroyed by Stealth Rock and even weaker attacks hurt due to the lack of survivability. Without Dragon Pulse, Lati@s can switch in and kill you. Without Focus Blast, Tyranitar beats you. Without Earthquake or Focus Blast, Heatran walls you all day long. Without Flare Blitz, you don't have a hope of getting past
the pink blobs Blissey (Chansey can still Wish+Protect or Softboiled off the damage). Depending on what they come in on, it is very easy to guess those two moves. For example, if they ever have a choice and come in on SR, they almost certainly have Roost. If they come in on one of the conditional checks, then they carry a move for that particular check. Now, I would like to revisit the definition. "
If the Pokemon in this rank have any flaws, those flaws are thoroughly mitigated by their substantial strengths." Charizard Y has one strength. It hits hard. It has subpar Speed, limited bulk (78/78/115 is pretty poor uninvested), and a crippling Stealth Rock weakness combined with a need to switch in and out to fulfill its role. The combination of these factors results in it being worn down very quickly. Due to the combination of these factors, I believe that Zard Y does not fit the S rank definition and should be moved down to A+.