I would argue that Kelly was an it girl for 2002, she had the best selling single of the year and was all over American media with Idol. Iggy would've been an it girl for 2014 too.
RIP at 2011 and 2012 tho When was the Stripped era for Xtina? She should be an it girl for that year too.
1999: Britney Spears
2000: Britney Spears, Christina Aguilera
2001: Jennifer Lopez, Alicia Keys
2002: Ashanti
2003: Beyonce
2004: Lindsay Lohan, Ashlee Simpson
2005: Mariah Carey, Kelly Clarkson
2006: Nelly Furtado
2007: Britney Spears
2008: Miley Cyrus, Rihanna
2009: Lady Gaga, Megan Fox
2010: Lady Gaga, Nicki Minaj, Ke$ha
2011: Kim Kardashian, Adele, Nicki Minaj
2012: Jennifer Lawrence
2013: Miley Cyrus, Beyonce
2014: Ariana Grande, Iggy Azalea
2015: Taylor Swift, ???
Notes:
2011 - I didn't list Adele because although she demolished the charts, I don't feel like she reached that fever pitch of media domination. EDIT: I changed my mind.
2012 - I didn't consider Carly Rae. CMM may have been the IT song of 2012 but Carly was definitely not the IT girl.
I feel like Rihanna was big in 2012 too with Unapologetic dominating.
It didn't release until November. She was obviously big as she is most years, but wasn't the girl who dominated media discussion and had everyone talking.