Chansey outclasses Blissey, her evolution, and is ranked OU whilst Blissey is UU.
Not sure about that one, but competitively, Dusclops outclasses Dusknoir because of its superior bulk; it has almost the same moveset too. But anyways, it's pretty bad.
Porygon2 is just as good as PorygonZ when it comes to rankings; both of them are UU.
However, they have very different roles; the former is a wall / tank whilst the latter is a boosting sweeper that abuses pretty well Nasty Plot and Agility, called AgiliPlot.
Doublade doesn't technically rank higher than Aegislash; whilst the latter is banned to Ubers, the former sits in RU, mainly due to the fantastic bulk Eviolite gives it. Doublade is kinda the new Dusclops now; but Aegislash is far better than Dusknoir, no comparison can stand.