Nicest is probably Kelly.
Meanest is a harder one to answer. There are different types of mean: there's actively aggressive meanness, like beating a woman in a bar while quite possibly shouting racist epithets at her (Cheryl), but there's also a less overt (yet just as sinister), passive-aggressive meanness, which is often characterized by feigned niceness and subtle, more indirect aggressions in order to avoid blowback. The latter is best exemplified by the likes of Taylor Swift. Apples to oranges.
