Ok. HOW is Mega Rayquaza NOT S/S+ rank?!?
It was literally BANNED FROM UBERS. THAT IS HOW OP IT IS.
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Mega-Rayquaza is the FIRST item-unrestricted ~800 BST pokemon. (Most other Ubers are ~700, and the ones that get close to MRay's BST have item restrictions, allowing it to exploit its large stats alongside a good item such as a life orb/leftovers/etc., while others are stuck without any items) Top that with a ridiculously OP ability in Delta, reducing its 4x ice weakness to only 2x, and nullifying its rock weakness. Not to mention it's good offensive typing and a moveset with a wide array of powerful moves.
Mega Rayquaza: A -> S/S+
Also non-mega Rayquaza is completely outclassed and should be moved accordingly (Not in same ranking as MRay, also maybe move down to A- because of opportunity cost)
Let me tell you a thing or two, about Mega Rayquaza in Ubers, and Mega Rayquaza in VGC2016.
Mega Rayquaza was overpowered in Ubers for a couple of reasons that don't apply in doubles:
The sheer power of Mega Rayquaza's sweeping ability is a result of Ubers being a single battle format. This meant that, if Rayquaza kills something, the turn is over and is reset when a new Pokemon comes in. What that means is there is little stopping Rayquaza from killing mons after mons. In doubles, Rayquaza must be mindful of its target selection, and the potential revenge kill from the target's partner. Worst situation is Rayquaza attacks a Protected target and gets bopped by the partner. That kind of stuff doesn't exist in singles. He kills your mon, and he kills your mon again. This kind of sweeping threat caused all playstyles but hyper offense to crumble, which caused games to be extremely linear: whoever manages to set up Mega Ray first wins the game. If you thought Primal Groudon was centralizing enough, Mega Ray just takes it to the next level by invalidating defensive playstyles because things die over and over again too quickly with barely any room to counter it. At least Primal Groudon allows many different playstyles to exist.
Furthermore, his opportunity cost is zero in Ubers. There is no Uber limit, and Mega Ray was clearly the best mega.
VGC2016 brought several new issues for Mega Ray. Aside from the aforementioned singles/doubles difference, Mega Ray's high focus on single target damage combined with its relative frailty (make no mistake, 105/100/100 is respectable, but between Dragon Ascent drops and its weaknesses to the ever so common Dragon, Fairy, and Ice attacks can cut its life short). Rayquaza often has to carry Protect to defend itself from enemy focus, which limits its coverage compared to in Ubers (and Ray gets basically every coverage option but it only gets like 1 slot to use them). Its signature set up sets in Ubers run into issues in doubles, where Speed control is common. ExtremeSpeed can only take it so far against Trick Room, for example. Disruptions like Follow Me and Fake Out also open Rayquaza up to counterattack. Finally, it eats up both the restricted and mega slot on your team, which is something that perhaps separates Mega Ray from the likes of Mega Kanga and Mega Salamence, and it means Rayquaza has to work overtime to cover up for team composition deficiencies caused by this stacked restriction.
That's not to say Mega Ray isn't good, and it's possible that after the hype train crashed after Day 1 that Mega Rayquaza will eventually find its way out of A and into A+. It's still a strong mon, but at this rate it just seems very unlikely it'll ever reach Groudon tier.