Boy, that's a tough call. Alexis has been the champ for awhile in that division, and while her ass isn't quite what it used to be, and Gracie is probably the #1 contender...I still think the champ has some fight left in her, so I'll go with Alexis, but barely.
But, that's just my vote for the "best ass". My bet is Gracie's the better fuck.