Katy Perry... I find her too gimmicky and not much of a singer, though I'm far from trendy
I like some songs from Fergie like "Glamorous" and "Clumsy" and enjoy Natasha Bedingfield. Would say Duffy, though she's more of a '60's throwback artist and don't think she's as popular compared to Katy Perry & country crossover Taylor Swift.
Duffy is definitely popular... best selling 1st album for 2008 (4,5 million sold), and 4th best-selling album for 2008.