Subjective of course, but i think aya has a slightly overrated ass. (I mean, fun bouncy ass, but not the greatest). But a significantly underrated face. Well, in this review Cyrus is pretty effusive, so he doesn't underrate her face. Correctly rates her face.
My subjective pick for currently working asses:
Not necessarily 100% organic: Gisella (new ny stint starting soon it seems).
Organic-only: Iris via bookersol.