Alright, so although I've kind of dropped VGC doesn't mean I can't keep up with the metagame. Rayquaza's main job is to nullify Primal weather and either nuke things with Mega Evolution + Dragon Ascent, or pick off weakened targets with ExtremeSpeed. While most go fully physical, there is merit to a mixed set. However, a mixed set should still lean more heavily towards the physical side because that's where the most payout is. Here's a comparison of Rayquaza's best Physical and Special options.
Air Slash vs. Dragon Ascent
Dragon Ascent is much, much better. Even though it lowers defences, its raw damage output and perfect accuracy is what puts it ahead of Air Slash.
0 Atk Life Orb Rayquaza Dragon Ascent vs. 4 HP / 0 Def Primal Kyogre: 136-161 (77.2 - 91.4%) -- guaranteed 2HKO
0 SpA Life Orb Rayquaza Air Slash vs. 4 HP / 0 SpD Primal Kyogre: 55-64 (31.2 - 36.3%) -- 50.6% chance to 3HKO
Dragon Claw vs. Dragon Pulse vs. Draco Meteor
Of all these, Draco Meteor is probably the best for a physically-biased mixed set purely because it lets you eliminate opposing Rayquaza without harming your physical offenses. Arguably, a Dragon STAB is not needed because Dragon Ascent is so ridiculously strong and other coverage moves are more valuable.
For a specially-biased set, instead of Dragon Claw I suggest something that covers a major threat. The only relevant things that Dragon attacks hit anyways are Palkia, Giratina and other Rayquaza, and Dragon Ascent already does titanic damage.
While it's good on paper as Xerneas deterrent, Dragon Ascent is better and less niche.
252 Atk Life Orb Rayquaza Dragon Ascent vs. 0 HP / 0 Def Xerneas: 153-183 (76.1 - 91%) -- guaranteed 2HKO
252 Atk Life Orb Rayquaza Iron Head vs. 0 HP / 0 Def Xerneas: 138-164 (68.6 - 81.5%) -- guaranteed 2HKO
252 Atk Life Orb Mega Rayquaza Dragon Ascent vs. 0 HP / 0 Def Xerneas: 177-211 (88 - 104.9%) -- 25% chance to OHKO
252 Atk Life Orb Mega Rayquaza Iron Head vs. 0 HP / 0 Def Xerneas: 161-190 (80 - 94.5%) -- guaranteed 2HKO
Flamethrower / Overheat
This is where mixed Rayquaza is best. A Fire move lets it hit Ferrothorn, which proves especially useful on Kyogre teams. RayQuay comes in, removes the rain, roasts Ferro, retreats. Overheat is generally better because it lets you nuke other Steel-type threats like Mawile and Aegislash.
Other moves: ExtremeSpeed, Waterfall, Earthquake, Earth Power, Rock Slide
Most of Rayquaza's choice coverage moves are physical with the exception of Earth Power. ExtremeSpeed is super useful, outspending stuff like Bullet Punch and Prankster status. Waterfall lets it KO Primal Groudon with Desolate Land down. Earthquake is another Groudon and Steel-type killer if you opt out of a Fire attack. Earth Power is for when you want to hit Groudon through Wide Guard support, in the Harsh Sunlight (but even then Dragon Ascent is better). Rock Slide takes care of Ho-Oh, Thundurus, and Talonflame.
Physical Rayquaza: Dragon Ascent + ExtremeSpeed + Protect + Waterfall — the best in most situations
Mixed Rayquaza (physically biased): Dragon Ascent + ExtremeSpeed + Protect + Draco Meteor/Overheat/Earth Power — slightly more situational than fully physical but not bad at all
Mixed Rayquaza (specially biased): – Dragon Ascent + Draco Meteor + Flamethrower + Protect — worse then a physical set but still usable
Special Rayquaza: — Dragon Pulse + Flamethrower + Earth Power + Protect — rarely useful and one of the above sets is probably better