Moves
Dragon Ascent is necessary to Mega Evolve and it's Rayquaza's best STAB move, doing incredible damage to anything that doesn't resist it. Extreme Speed gives Rayquaza a powerful priority move that beats out Talonflame, Kangaskhan, and Prankster users attempting to hit it with Thunder Wave, and it also mitigates Mega Rayquaza's vulnerability to speed control from the opposing team. Draco Meteor is a secondary STAB option that does massive damage even with no investment, hitting other Dragon-types and OHKOing uninvested Primal Groudon while also letting Mega Rayquaza work around Intimidate a bit. Overheat hits Steel-types like Mega Mawile, which otherwise checks Rayquaza, while OHKOing Ferrothorn and having a chance to OHKO Aegislash. Waterfall is a much more reliable move against any Primal Groudon variant and doesn't risk missing like Draco Meteor. However, this is generally only used when Mega Rayquaza is paired with Primal Kyogre, as Mega Rayquaza benefits from the rain with Waterfall. The utility diminishes greatly without Primal Kyogre, since unlike Draco Meteor, it can no longer hit Primal Groudon if sun is up. Protect is important in the last slot to let Rayquaza scout out for threatening moves, since it will often be targeted due to its average bulk and its dangerous power. It also lets Rayquaza safely Mega Evolve to get past its average Speed before Mega Evolution.
Set Details
Maximum Attack and Speed investment maximize Mega Rayquaza's damage output while ensuring it's as fast as possible. The leftover EVs are placed in Special Attack for Draco Meteor and Overheat, though they can be moved to Special Defense if you're running Waterfall. A Speed-boosting nature is important to take advantage of Mega Rayquaza's fantastic Speed tier and prevents it from being outsped by Timid Primal Groudon and Primal Kyogre on the turn it Mega Evolves. Naive lets Rayquaza handle priority moves better to avoid being revenge killed as easily, and guarantees that Weavile can't OHKO with Icicle Crash after one round of Life Orb damage if Strong Winds are active, as well as surviving Double-Edge from Jolly Mega Kangaskhan after one round of Life Orb recoil. This all assumes that Mega Rayquaza hasn't already suffered Defense drops from Dragon Ascent, though. Alternatively, Hasty lets Rayquaza switch into Primal Groudon's Eruption and Primal Kyogre's Origin Pulse more comfortably, which is useful since it's one of few Pokemon that can comfortably switch into both. Unboosted Dazzling Gleam from Timid Xerneas also has a much lower chance of OHKOing Hasty Mega Rayquaza after one round Life Orb recoil. A Jolly nature is only used with Waterfall, since the Special Attack investment is not important in this case.
Life Orb capitalizes on Mega Rayquaza's offenses and is important for helping it secure OHKOs, such as with Draco Meteor against Primal Groudon and with Dragon Ascent against uninvested Primal Kyogre. The extra power also lets Rayquaza hit Pokemon hard even without Mega Evolving so that it can comfortably be brought into battle with a different Mega Evolution. Focus Sash has a noticeable power drop compared to Life Orb but can be useful for letting Mega Rayquaza serve as an emergency check to boosted Xerneas due to Dragon Ascent's power and Extreme Speed's priority. It also functions as a safe buffer that lets Mega Rayquaza clean more effectively late-game after the rest of the team has weakened the opposing team while also mitigating the issue of Mega Rayquaza falling to a single strong hit after repeated Dragon Ascent drops, which can be critical in such a fast-paced format.
Usage Tips
Mega Rayquaza's strengths lie in the ability to punch holes in everything; it has the power and Speed to threaten the entire metagame after Mega Evolving, so capitalizing on that is vital. Mega Evolving early can put immense pressure on the opponent, and even Xerneas is greatly threatened if Rayquaza manages to Mega Evolve before it can come in and set up, since Xerneas can be KOed after very little prior damage. Likewise, saving Rayquaza until late-game is effective, since it's very hard to stop once a team has been weakened. Rayquaza doesn't need to try to sweep the entire team though; sometimes one crucial OHKO is all it needs to have done its job successfully.
While Rayquaza's bulk is somewhat average, its typing coupled with Air Lock and Delta Stream makes it one of the best Primal switch-ins in the format, so don't be too afraid to switch it into dangerous moves if the opportunity presents itself. The freedom of being a Mega Evolution with the ability to hold any item also means Mega Rayquaza can freely be brought along even with a secondary Mega Evolution without compromising its strengths much. Non-Mega Rayquaza still has dual 150 offenses, so it's more than capable of threatening teams with a Life Orb without Mega Evolving if the second Mega Evolution is desired.
Team Options
Primal Kyogre is one of Mega Rayquaza's most popular partners, as Mega Rayquaza lets Primal Kyogre fire off Origin Pulse and Water Spout more freely and put huge pressure on Primal Groudon attempting to switch in out of fear of Rayquaza coming in to negate its harsh sunlight. Mega Rayquaza also helps beat Primal Kyogre's best switch-ins such as Ferrothorn and Amoonguss. However, Air Lock will also negate the rain boost from Primal Kyogre, so be aware of the various weather interactions when both are on the field. Xerneas appreciates Mega Rayquaza's ability to threaten Primal Groudon and Steel-types, giving Xerneas less trouble when it's attempting to sweep and also cleaning up late-game if Xerneas can't finish the job. Xerneas also makes up for Mega Rayquaza's lack of spread damage while also beating Mega Salamence. Ho-Oh handles Xerneas very comfortably for Rayquaza due to its monstrous Special Defense, and it also appreciates Delta Stream to lessen the damage of Rock Slide.
Tailwind support from Talonflame or Crobat helps Mega Rayquaza outspeed Mega Salamence and boosted Xerneas. Priority Brave Bird from Talonflame is also useful for helping bring something into KO range for one of Mega Rayquaza's attacks and can put more pressure on Xerneas, while Crobat offers Taunt support to prevent Xerneas from setting up and keep Smeargle in check. Both also benefit from Delta Stream to take Rock-, Electric-, and Ice-type moves more comfortably. Likewise, Landorus-T appreciates Delta Stream to reduce the threat of Hidden Power Ice and provides utility in the form of Intimidate while being able to freely use Earthquake alongside Mega Rayquaza. Steel-types like Aegislash and Ferrothorn threaten Xerneas, while Mega Rayquaza threatens Primal Groudon for them. Thundurus does a great job at supporting Mega Rayquaza with Prankster Thunder Wave to cover Xerneas and Salamence and has Taunt to deter Smeargle, while Mega Rayquaza's Delta Stream removes its weaknesses to make it more difficult to beat.
Redirection from Togekiss or Amoonguss can be deadly against teams that lack many answers to Mega Rayquaza and notably prevents Mega Salamence from taking it out reliably. Amoonguss handles Xerneas with Spore and Clear Smog, whereas Togekiss benefits greatly from Delta Stream and is also immune to Dragon-type moves aimed at Rayquaza. Mega Gengar functions as a great secondary Mega Evolution that has a better matchup against Xerneas. Both Gengar and Rayquaza are still good without Mega Evolving, so bringing both is not overly limiting either. Weavile is also popular as a Fake Out partner, and it also provides Feint support while threatening Mega Salamence with Icicle Crash. Feint lets Rayquaza safely break through anything trying to Protect, and Weavile is the only Feint user that outspeeds Mega Rayquaza, which allows it to use Extreme Speed more freely in front of Quick Guard users. This also provides valuable, albeit minor, chip damage to secure KOs against threats investing in bulk to otherwise survive a hit from Mega Rayquaza, and it can turn rolls into safe KOs. The combination of Mega Rayquaza and Weavile is also one of the few pairs that can stop Mental Herb Cresselia from setting up Trick Room.