Rayquaza, in my opinion, has fallen out of grace in ths generation. As a sweeper, it faces stiff competition from the best pokemon in the game: Arceus. Rayquaza's bulk does not do it any favors. It will usually lose over 50% of its health from most neutral hits. Rayquaza has always underperformed in most of the teams I used it in. I could not understand why I would use it over other Dragons - Dialga, Palkia, Giratina-Oriin, Lati@s. Unlike Rayquaza, who is one-dimentonal, i.e, an offensive threat, the other Dragons can greatly contribute to the team's defensive synergy.
The release of Genesect and Multiscale Lugia is the nail in the coffin for Rayquaza. Genesect naturally outspeeds Rayquaza and can revenge kill all its sets. Rayquaza Mixed set can't do anything that a simple Life Orb SubRoost Ho-Oh cant do.
You are free to disregard what I have said. After all, our experience in using a Pokemon is likely to be different.
Wait what ?_? Rayquaza still is a powerful sweeper with access to wall breaking abilities matched by few, if any, pokes in the game! Multiscale Lugia gets OHKOed after Stealth Rock by Draco Meteor + ExtremeSpeed. Those lacking heavy defense investment face getting OHKOed by Dragon Dance boosted Outrage after Stealth Rock!
Genesect is pretty big, but its not impossible to play around. Tailwind lets Rayquaza pounce right around. Using Thunder Wave or Substitute lets you land a huge hit on Genesect which prevents it from revenging you. Scarf Palkia was a huge issue earlier, but that didn't stop this guy from dropping significantly in usage did it? You could even use Yache / Haban Berry if your desperate ;_;. Not to mention Gene has to switch in on Dragon Dance to kill Rayquaza, if it switches in Draco Meteor or Fire Blast or Earthquake or V-Create its as good as dead. It can't handle SD Rayquaza either, who literally mauls the bug with ExtremeSpeed (granted it needs to be weakened a good bit before being OHKOed).
Rayquaza's bulk doesn't do it favors, the only one who has any favors getting done for it is Arceus, but that guy lacks Dragon STAB. It lacks that Draco Meteor. The ability to destroy Ferrothorn in rain, boost its speed... they don't even have the same counters seeing as Dragon Dance Rayquaza smashes right past all of Extremekillers. Under the thinking a sweeper needs bulk to setup on its own would make Darkrai (although this guy does has Dark Void to help) and Mewtwo completely unviable, but that's not the case is it?
It doesn't contribute to synergy but it boasts things other Dragons could only dream off. Can Dialga fry Ferrothorn in rain? Can Palkia boost its Speed outside of Choice Scarf? Can Palkia even beat Kingdra in the rain? Can Latios spam +1 Outrage that OHKO 252 HP Arceus forms barring Steel who gets OHKOed by Earthquake anyway after Stealth Rock? I will agree life has gotten hard for it, but it is definetly usable.
EDIT: @Anikrahman, all right so you just can't get it to work like how Mewtwo fails for me. I know how ya feel then lol :P. If your having trouble finding a spot for Rayquaza I recommend using it on a team that wants a sweeper (it provides an easy win condition IMO), or a wall breaker, or both (Mixed Dancer).