Rayquaza @ Life Orb/Lum Berry/Weakness Policy
Ability: Air Lock (becomes Delta Stream)
EVs: 252 Atk/4 Def/252 Spd
Adamant Nature
-Dragon Ascent
-Earthquake
-ExtremeSpeed
-Dragon Dance/Swords Dance
short version: ggwpgg
detailed version: Ok so this will probably be the most common Rayquaza set, and for good reasons, as it simply gives no chance to any team if he can setup.
Dragon Ascent is mandatory as you can't mega-evolve without it. It is pretty much like a Flying type Close Combat so it's not like it is a bad thing at all.
Extreme Speed is good priority, that even on DD sets, where it can seem rendundant, lets you bypass other priority such as Yveltal's Sucker Punch, as well as faster scarfers(Darkrai and Skymin). On Swords Dance sets it can OHKO many scarfers and faster Pokémon after a boost, making it a mandatory choice.
I think that Earthquake is better than V-Create, as Dragon Ascent already obliterates Ferrothorn and Scizor, while Earthquake deals with Heatran, Diancie, Arceus-Rock, Tyranitar and Zekrom, Pokémon that would otherwise stop a V-Create set. More important, it lacks any undesiderable side effect; seriously, one of the things I hated about Ray was the V-create drops that forced him out, preventing it to sweep. Dragon Dance is gud because it lets you outspeed more stuff, Support Arceus in particular, but the pumped up Extreme Speed makes Swords Dance just as good against almost everything except Arceus.
About the item choice: Life Orb and Lum Berry are obvious, one gives extreme amount of power, while the other makes setting up on stuff easier. Weakness Policy may seem odd, but it actually pairs pretty well with Delta Stream; despite the Delta Stream, the game still acknowledges Rock type moves as supereffective, not to mention Ice moves. Delta Stream allows Ray to do stuff like taking Ice Punch from Mega Blaziken and Ice Beam from Scarf Kyogre if at full HP, at which point the game might as well be considered closed if Ray sets up in the same turn.