Taylor for sure. Her only competition is Adele. Gaga comes 3rd with the remaining impact of The Fame Monster and Born This Way. To close the top 5 we have Rihanna with her massive singles and arguably Beyoncé with her acclaim.

Madonna, Beyoncé, Katy

Taylor Swift, of course.

EDIT: I didn't read the "pick your top five" part and only voted Taylor

Taylor > Gaga > Katy > Beyonce > Rihanna