Ah Ubers.
Dialga
Is this standard set? I beleive it is, I think you may have got it from smogon.
Either way this looks good though personally I would abuse dragon tail instead of roar as it hits giratina.
Arceus
Looks good to me, great counter to weather starters.
Rayquaza
Try my set.
Rayquaza @Choice Scarf
Trait: Air Lock
EVs: 4 HP / 252 Atk / 252 Spe
Naive Nature
-V-Create
-Outrage
-Extreme Speed/Earthquake
-Draco Meteor
This should work much better as well as give your team more speed.
Mewtwo
it doesn't have what you call bulk at ubers standards at least, I have never found a calm mind set very helpful.
So for you I suggest running Thunder it counter Kyogre.
I would also replace Ice Beam for Flamethrower as it gets rid of Pokemon trying to set up hazards.
Finally you may want to try out Choice Specs on this guy lol, it hits everything like a truck.
Reshiram
:( you have 5 special attackers, this needs changing so how does Ho-oH sound?
Ho-oH @leftovers
Trait: Regenerater
EVs: 244 HP / 252 Atk / 12 Spe
Adamant Nature
-Sacred Fire
-Brave Bird
-Earthquake
Recover/Flame Charge
Regen and recover both help make up for its unfortunate SR weakness and possible recoil.
But apart from that it plays a very similar role to Reshiram just a physical tank version ;)
Manaphy
Okay this Pokemon is only really Ubers for the possible rain support.
This is because of Hydration and Rest, then pair it up with Tail Glow that gives it +3 attack...
You have non of them attributes therefor Manaphy becomes almost useless here.
Try Palkia it has better coverage and stats.
Palkia @Choice Scarf/Specs
Trait: Pressure
EVs: 4 HP / 252 SAtk / 252 Spe
Modest Nature
-Special Rend
-Hydro Pump
-Flamethrower/Ice Beam/Aura Sphere
-Thunder
My favorite Pokemon and set I have ever used probably (excluding PP stall Lugia)
Finally I'm finished, allot of these suggestions really need to be made but I hope this helps
