Lana Del Rey.
She wasn't being shoved down our throats and her songs weren't clogging up the Top 40, but she achieved SO much this year. She was picked up/sponsored by huge companies, covered the most prestigious magazines all over the world, and has a huge online fan base which eventually evolved into album sales that rival a lot of the top pop girls of today.
I guess people could argue Nicki Minaj since she was the one in the public eye, and Nicki achieved heaps this year, too. I just feel that Lana's achievements were much bigger and exceeded many more expectations than Nicki did.