Manaphy

Manaphy is often left in the dust in lieu of its fellow Uber Water-types Kyogre and Palkia. Manaphy absolutely needs rain support from Kyogre to do anything in Ubers, and both Kyogre and Palkia have much better offensive stats as well as movepools. Furthermore, Manaphy also has to compete with fellow rain abusers Kingdra and Kabutops, who both get Swift Swim to boost their Speed stats to astronomical levels. (Kingdra even gets Dragon-type STAB like Palkia.) Faced with these competitors, one may think that there is no reason to use Manaphy at all.

However, Manaphy does have a few tricks up its sleeve. Hydration, when used with Rest and rain support, allows Manaphy to instantly heal itself and remove any status condition, which no other Pokemon in Ubers can do. It may not be as immediately threatening as either Kyogre or Palkia, but its access to Tail Glow means it can power up faster than either of them. Manaphy is also much bulkier than Kingdra and Kabutops.

Its movepool is somewhat limited compared to its fellow Ubers and rain abusers, but what it has is just enough to make it a decent threat.