if i may share my opinion. i only use two sets of rayquaza: swords dance and mixed set.
i agree with the guy up there who said that SDquaza be paired with arceus normal (sorry can't quote as i am using my phone). the idea of using the two must be a little redundant to some but please do note that the two perform very different roles. rayquaza has a higher attack stat but has a weaker extremespeed and is slow while arceus is the exact opposite. so you might just throw out arceus first to sweep. wrong. i would go out for rayquaza first because it has a smaller number of checks. all things that wall arceus die to rayquaza. well it's like the double dragon strategy between quaza and mence except you use extremespeed. so all in short terms, wall break with one pokemon and sweep with the other.
as for mixquaza, this is where the double dragon strategy comes in best. as usual, rayquaza's main role is to destroy walls to open up a sweep with the other pokemon.
so it still may seem that rayquaza is outclassed by some of the pokemon who can sport the same role but fear not, it's wallbreaking prowess is still unrivaled.
If Slaking gets another ability other than Truant(and Slow Start), it would be Uber with that monstrous stats.