2008: Katy Perry
2009: Lady Gaga
2010: Ke$ha
2011: Adele
2012: Carly Rae Jepsen
2013: Miley Cyrus
2014: Iggy Azalea
Cristina Perri is great, but not for nothing, she's never even had a top 10 hit! "Jar of Hearts" reached #17, and "A Thousand Years" and "Human" both peaked at #31. She was a great breakthrough act in 2011, however, not the it girl.
I think this list is leaning towards one-hit wonders/short-lived careers. Only the 2 most recent artists are still relevant. And IggySZN seems to be coming to an end.