With the recent fact that Shiftry is now RU, i would say that Mesprit & Uxie should go to A or even A+
I judged that arguing about Mesprit is kinda pointless, people on this thread already showed that this thing was extremely versatile and viable to deserve A/A+
So, i guess i'll argue more about Uxie.
Uxie's bulk is probably the best bulk in the entire NU Roster, 75/130/130 makes it extremely tough to OHKO, which is very helpful for a support.
But the reason Uxie should be A is for its movepool, Uxie's access to Yawn, Thunder Wave, Toxic, Trick, or Memento allows him to easily cripple a lot of pokemons in NU.
Also, his access to support moves such as Reflect, Light Screen, Heal Bell makes Uxie the best support available in NU in my opinion.
With 95 Speed, Uxie will outspeed a large amount of lead in NU, and can go away with U-Turn, or simply act before your opponent, it's always helpful.
Uxie is also able to work as a very nice Weather Support, with access to both Sunny Day and Rain Dance.
It can even be played offensively with a Sub Calm Mind set.
The only thing that prevent Uxie from being S Rank or even RU is his lack of reliable recovery, and his typing that isn't really helpful in Gen 6, with the prevalence of Knock Off.