Rayquaza

Rayquaza is one of the most dangerous restricted Pokemon in the format, boasting nearly unparalleled offensive prowess thanks to its Mega Evolution. Mega Rayquaza's dual base 180 offenses are the highest combined offenses in the format, and with its diverse movepool, it can threaten everything in the metagame. A high base 115 Speed complements Mega Rayquaza's offenses well, only leaving it outsped by a few unboosted threats such as Mega Salamence and Weavile. As the only Mega to have the freedom of choosing its item, Rayquaza is also easy to fit on teams with another Mega such as Kangaskhan or Gengar, and it's one of few viable Mega Evolutions to be a threat without needing to Mega Evolve. Dragon / Flying is a great typing as well, allowing Mega Rayquaza to switch into most attacks from both Primal Groudon and Primal Kyogre. Both Air Lock and Delta Stream are incredibly useful abilities that further increase its ability to effectively check both Primals by allowing to shift the weather war in its favor, and unlike the Primals, Mega Rayquaza isn't hindered greatly by the loss of its weather. Delta Stream also lets Mega Rayquaza support its Flying-type teammates, reducing the damage they take from Rock-, Electric-, and Ice-type moves.

However, Mega Rayquaza suffers from a few flaws that keep it from being omnipresent to the same degree as Groudon, Kyogre and Xerneas. While Dragon Ascent is incredibly dangerous with few viable Steel- or Rock-types to switch into it, the stat drops and Life Orb recoil make it much easier to KO Mega Rayquaza despite its respectable bulk and numerous useful resistances. A Fairy-type weakness also makes Xerneas a big threat for Rayquaza if it can't Mega Evolve before Xerneas comes in. Rayquaza's base 95 Speed before it Mega Evolves is solid among most restricted legendaries, but it still leaves it outsped by key threats including Salamence, Timid Xerneas, and Jolly Mega Kangaskhan. This sometimes forces it to rely on Protect to safely Mega Evolve, which can be taken advantage of by the opponent. Rayquaza is also always used on a team with the intent to use its Mega Evolution, so while it's no less dangerous than Mega Kangaskhan or Mega Salamence, the restriction of two legendaries limits it somewhat in terms of team archetypes it can fit on.