I mentioned Mew not because it's similiar to Manaphy, but because it's simply not used at all in the Uber metagame. That doesn't mean it belongs in OU, though. Same with Manaphy; just because it cannot contend with the likes of Kyogre and Palkia does not mean it should be sent to OU.Isn't Mew in uber for an entirely different reason though? From what I've seen in using Manaphy personally, fighting against manaphy and reading about his use (I think there's a warstory with him in the archive)he's not that much of a threat.
He's incredibly predictable and easily taken down. Most pokemon ,especially Gengar, with a super effective attack can take him out. Also, Manapy requires that he be switched in meaning the majority of the time you won't be facing a manaphy at full health.
Battles usually go something like this,
*rain dance*
Manaphy switched in, X pokemon uses X attack depleting 1/4 of his HP.
Manaphy uses tail glow, X pokemon switches out.
X pokemon is faster uses SE hit, manaphy faints. Occaisonally X pokemon is slower and is hit with NVE or nuetral damage and then manaphy faints.
No one likes to switch out because tail glow is wasted and it defeats the purpose of using him in the first place.
Even if uber manaphy did carry grass knot that's why I mentioned Palkia. I just can't see Manaphy as uber.
Damage: 230 - 270
Damage: 67.45% - 79.18%
Assuming I calculated this correctly, that's what a max special attack (positive nature) Gengar will do to a min/min Manaphy with Thunderbolt. Since most Gengars go for +Speed and not +Special Attack, it's not going to be a kill even after Manaphy lost 25% of it's health. And while I haven't calculated it, I'd assume +2 Manaphy's STABed Surf in the Rain would OHKO Gengar.
Manaphy's defenses are not to be overlooked; it can take a beating, especially with it's mono Water typing.