Problem about that "combo" is that you have to make a Rain dance team to support Manaphy's Hydration rest, Kyogre can't help it OU. Manaphy only has 3 usable physical attacks (Waterfall, U turn, and a normal move) so it's easier to predict its moveset. Jirachi and Celebi both have those stats and no one is complaining. Also so many other pokemon have Nasty Plot/ Calm mind, this pokemon just doesn't seem good enough and unpredictable enough to be considered uber.
Congrats. Seriously, I don't understand why its so hard to understand that you can't use the performance of Manaphy in Uber to ban it from OU. Anyway, Manaphy
was emperically tested for a few months in the OU environment. After this period of testing, Manaphy was banned to Uber.
Tail Glow made it exceptionally powerful, and unlike the "other" pokemon with a Nasty Plot set, Manaphy can actually withstand a few hits while having enough speed to tie with Zapdos.
I mean, look at the other Nasty Plotters. Azelf dies almost as quickly as Alakazam. Slowking can't sweep. After factoring Togekiss's lower HP, Togekiss only ties with Sp. Def and is worse in physical defense and has much worse speed.
Comparing Calm Mind to Nasty Plot is like comparing Bulk Up to Swords Dance. The opponent only has to take 1 hit from a nasty-plotted attack in order to "make up" for the turn setting up. On the other hand, you require 2 hits after a Calm Mind. If you calm mind twice, you need to attack for 2 turns. Essentially, 2 turns of setup vs 4.
Further, Rest / Hydration was a very broken set. Bronzong easily set Manaphy up while negating its only weakness to fire.
Right now I'm posting counters, for it.
No you are not. A counter can switch in on any of Manaphy's attacks and come out on top. Manaphy doesn't run from Electivire, Infernape, or Sceptile. It kills them with surf / ice beam.
And Why do you people keep on saying, "Oh noes Hydration rest combo!", you guy attack as if people can't predict, we're stupid and just dumb down against a Manaphy and will let it set up on us. Geez, it takes turns to set up and we can set up too.
Anyways:
Max Sp atk Porygon-Z Life Orb Adaptability
Tri Attack does 40.59% - 47.77% to a Max HP 178 Sp. DEF Manaphy neutral nature
Thunderbolt does 64.36% - 75.50% to a Max HP, 178 Sp. DEF Manaphy
neutral nature
That is not a counter. Porygon Z will die from Surf before it can kill Manaphy.