Charizard Y is hands down one of the best, if not the best mega right now. It straight up forces a kill versus nearly all common defensive cores found on offense, notably those with Mew, Celesteela, Landorus, Magearna and/or Tangrowth and can easily pressure Pokemon that can take a hit such as Zygarde and Garchomp just by clicking Fire-type move or with potential HP Ice coverage. The WCoP match with blunder vs reyscarface shows that if Zard Y has a decent matchup, it can get a kill every time it comes in. Dugtrio even prevents blunder from risking Focus Blast on Heatran later in the game.
In addition to its breaking power, the current rise of Dugtrio makes Charizard Y that much better. Z-Crystal Dugtrio traps everything found on fatter teams that Charizard Y can't break past, which is most notably Chansey and Toxapex. After one or two of these have been dealt with, Charizard Y can proceed to get a kill every time it comes in. These two WCoP matches demonstrate how devastating Dugtrio + Zard Y can be versus stall. welliou vs starmaster and TDK vs ict (note that Screech Dugtrio should have been able to cleanly trap Chansey without the crit).
Lastly, Zard Y isn't even that hard to support right now despite its 4x weakness to rocks because of Mew's unprecedented Defog support. Zard Y + Mew is another extremely common core seen in WCoP whenever Zard Y is used, and for good reason. Mew is straight up the best Defogger in the tier right now, and it pairs quite nicely with Zard Y. It reliably pressures Zygarde, Medicham, and Mawile, all of which offensively pressure Zard. If Volcarona sits at A+ I see no reason that Zard Y shouldn't sit at A, as Zard doesn't require a boost to do its job, and does not need to worry nearly as much about getting revenge killed.