Darkrai to S rank.
Reasons:
--Life Orb item option with Dark Pulse + Focus Blast. Basic, amazing coverage and damage that is really good for Ubers standards. Not weather reliant.
--Great options in Thunder and Taunt, Nasty Plot and Substitute (these are meh options IMO), and even Sucker Punch is viable for Mewtwo. This leaves it fairly unexploitable. It can tailor to your team, and the team can tailor to it. (Offensive teams anyway, and offense is probably the best playstyle.)
--Faster than Arceus and Lati@s.
--Dark Pulse can flinch. Thunder can para. Focus Blast can lower sp. def. Cool possibilities. Risk reward is great with this mon. Your only worries are missing!
--Dark Void + being fast means you are putting something to sleep nearly every battle. Darkrai essentially trades itself for at least one "kill" a game if played right, but on average it just does so much more than that.
--Dark Void + Bad Dreams damage <-- underrated.
--Dark Void + guessing games. Sleep Talk will always be an unreliable counterplay. Better off putting Lum Berry somewhere on your team. (Lum E-Killer gets mad props in my book.)
--Good switch-in to Arceus-Ghost and sometimes Giratina-O. Really. People like overusing these ghosts, so there will likely be one on the opposing team.
--Decent against Tyranitar if encountered in a lead match-up.
I disagree with rai moving up to S. This is because although rai is a big threat and needs to be prepared for, every team carries a check and is able to deal with rai in one way or another. It could be a revenge scarfer, a sleep absorber, what have you, but it stops rai none the less. It definitely needs to be accounted for when building a team, but it is not very difficult to deal with. In addition you said that rai is a "good switch-in to Arceus-Ghost" which is not the case as you must be wary of focus blast which deals a huge amount of damage, and if it's SD shadow force also deals big damage and has a chance of OHKOing at +2, while Brick Break guaranteed OHKOs at +2. Rai has to be very wary switching in to ghostceus and relies heavily on prediction. Also, what would be the point of moving it to S when it's already at the top of A+? It's basically already there, and moving it up two spots wouldn't make a very big difference. Darkrai is fine at the placing it's at now, since it already shows how big of a threat it is. So yeah keep rai where it is.