At the moment, Katy Perry. Runner up; Nicki Minaj.
And honourable mention to Kim Kardashian, who I must have fantasized about over 1200 times in the past 8 years. Kimmy's staying power with respect to stroke value is stupendous.
I am not a fake breasts fan but ever since she got her awful implants (You can see the scar under her breasts) and changed her hair color to dark - she got INFINITE times sexier. I look at her and see her as being Karen McDougal's daughter (and what a 3-some that would be)