I don't know what everybody thinks, but there is one thing I'll make clear: Manaphy was banned for its Tail Glow set (Tail Glow/Surf/Ice Beam/Grass Knot), not Hydrorest. Take this as you will.
^ This
I know alot of people in this thread have had no experience with Manaphy in OU, but I was around back then, so I'll tell some things I know about Manaphy, and how many of your 'statements' about Manaphy range from being dumb to being completely and totally wrong. After all, I don't know how you guys could argue for bringing Manaphy back in OU when many of you don't even know why it was banned in the first place.
Manaphy was the most threatening sweeper in the metagame. The reason it was banned was because it was WAY too centralizing. Back in the day, do you know how many Manaphy counters there were? Three. Calm Mind Blissey, CM Raikou, and an extremely bulky Ludicolo. Granted, there are more things that can switch into Manaphy, such as Empoleon amd Abomasnow. But can anybody tell me what's wrong with this list? All of these Pokemon are BL, aside from Blissey (but who runs CM anymore?). So the people that were talking about overcentralization, Manaphy did really cause this to a great degree.
What would Manaphy be like in OU? I'll go ahead and use the example of Timid LO Suicune. This thing is a real pain to counter, and even impossible to do at some times. Think of Manaphy as a faster and harder hitting Suicune. In order to beat the Tail Glow set listed above, you often would have to give up multiple Pokemon, and watch your Blissey get set up on. The overkill with Manaphy is not the rain (that is just a bonus); it is it's extremely good defenses. It isn't weak to any pritory moves either like Garchomp was. Has anybody tried taking down a Jirachi or Celebi, but using netural moves? CB Heracross *barely* manages a 2HKO on Jirachi. Taking down Manaphy is like trying to take down a Bulky Gyarados using only physical moves. Everybody uses Heatran and such as a check against Celebi and Jirachi. But, the challenge is, try finding a Heatran equivalent to beat Manaphy. It's pretty hard. Most of the things that manage enough power to take down this little pixie are slower, and will get destoryed by a Tail Glowed Surf. Water is one of the greatest attacking types in the game, and Manaphy manages to gain almost perfect coverage while using its great STAB move.
So for those trying to say "Manaphy is OU!", some of you really need to rethink your points. It isn't easily walled, it isn't easily killed either. Beating Manaphy usually meant getting two of your guys killed (Garchomp anyone?). What was Manaphy? Manaphy was one of the most powerful mid game sweepers in OU, and its goal was to weaken your team to get owned by something else, and it did a damn good job.
Manaphy might need to be tested at some point. Perhaps the metagame has shifted to a point that it might not be as broken. Let's see.