But... I think the criterion here should be that the best ass belongs to a woman whose booty takes her ranking to a whole new level. In other words, upon viewing from the front, you think she's OK looking. However, when she turns around your estimate of her immediately goes way up... along with a certain part of your anatomy.
Using that criterion, my choice remains Mya Diamond. She's curvy enough from the front, and she's got an exotic face, but she's got an OMG booty that just won't quit.
