Okay, after using Rain Dance for a while, I really think Politoed should go. Not Manaphy, not Kingdra, Politoed. Hell, I haven't even tried Kabutops, Ludicolo and other annoying Swift Swimmers for a while and from just Kingdra and Manaphy alone you can tell rain gives them way too much support. If you want a fun raining Kingdra set to use, use Specs Kingdra. It's got practically the same Special Attack stat as SpecsJolt or Specs Latias, but with rain-boosted Water-type attacks and STAB Dragon attacks. On top of that, you have 538 Speed. I'm quite sure most people would agree that Manaphy in the rain is an asshole. He is. I'm using my favourite CM Manaphy again, and all you have to do is just to predict Shaymin-S switch-ins (Pretty much the only special attacker that can OHKO you at +1) and Ice Beam it as it comes in. Then you're pretty much free. Calm Mind/Boil Over/Ice Beam/Rest is fun.
P.S. This is coming from someone who was heavily opposed to banning Damp Rock in 4th gen UU, heh