Hands down,
Meagan Reed; I always got a thrill seeing someone banging out her big bubble butt.
Another mention, Diana DeVoe (technically she hasn't retired from porn entirely, just from performing--she now directs with her husband). She was getting that enlarged MILF booty during her last two appearances on camera and she did her first (at least on-camera) anal scene in one of those appearances. I was really beginning to dig her, though I wasn't impressed with her initially.