The question, really, is this. Is a particular rain sweeper broken, while others aren't? Or does Politoed's support make several pokemon too much for the metagame? The thing about Rain being arguably broken in UU in Gen IV was that the likes of Ludicolo, Kabutops and Gorebyss were ALL so powerful under Rain and banning one of them would not have made a serious impact.
exactly you would have to ban them all wich isn't justifiable since they aren't "broken" by themself, but because Politoeds support boosts them so much that they get much more powerful than they would be without. This is why i would say IF we are going to ban anything it should be Politoed (or manaphy, because its imo the only thing that comes close to being broken under rain everything else can be handled by quite some things not to mention how good Nattorei works against rain).