Music wise:
Beyonce Knowles: She has to have the most overwought, repetitive, uninspiring and overall ANNOYING discography on the face of this planet. Every time I hear Single Ladies I want to stab my ears out. I don't get why people haven't realized yet that she's been releasing the same lead single over and over again off of each album.
Kanye West: Besides the fact that he's a douchebag, I can't fathom what's the big deal over his music. It's nothing special or out of the ordinary.
Coldplay: Dull, dull, dull. Not a song I've heard by them hasn't made me near comatose by the end.
2NE1: I don't get the current obsession over these girls. They have like one song I can think of that doesn't get extremely annoying after the first listen. They're talented but their music is just ugh.
DBSK, ARASHI, SUPER JUNIORS, most Asian boybands in general: It's even worse than the boyband craze that used to infest the West 

 even sadder is the fact some Western fangirls actually believe there's this huge "difference" between them. I just don't get it. 
 Talent wise:
Christina Aguilera:
Talent wise:
Christina Aguilera: Her voice has never sound even remotely good to me. When ever she opens her mouth, all I hear is gurgling and screaming not these "amazing vocals"  everyone keeps fawning over. At least her sales these days do tend to reflect on her recycled and mediocre music. 
Utada Hikaru: Hikki's great and all and has a really pretty voice... but in all honestly, I've heard better Asian singers. Her vocal technique isn't very good and her live vocals tend to be off.