Hmm... I would say that Kesha is definitely the worse artist out of all of those, but I don't know anyone who actually looks up to Kesha or tries to follow her, so even though she's a complete moron I'd have to say that she hasn't done any damage to youth culture.
Katy Perry hasn't done anything bad that I know of. She seems to just be a regular girl. Yes, she talks about drinking and partying, but she just sings about things that regular college girls do anyway. She hasn't had an effect on youth culture at all.
Lady Gaga has done a lot for the gay community. I personally don't think that's a bad thing. She's had the strongest effect on youth culture out of any of the artists that you mentioned, but like I said I don't think that encouraging homosexuals to have the same rights as heterosexuals do is a bad thing.
Marylin Manson - wtf? Does he even count? He hasn't come out with anything since 2000. The only song most younger kids know from him is "Sweet Dreams" and they just think he's a weirdo. He has no effect on youth of today.
Nicki Minaj - I think that I'd have to say that she's the worst of all of these. The other people either had no effect on youth culture or (in my opinion of course) benefited it, but Miss Nicki is definitely a negative influence. There are (sorrowfully) lots of people who try to copy her "baddest b****" attitude and style. She hasn't done much for society besides convincing ugly girls that they look like Barbie so, in my opinion, she is the worst.