I know it's not as reliable as reviews and some ladies with fantastic asses may not be on Eros or not want to pay the extra money to be listed specifically in that category, but it might be a good starting point for your search, correlated to TER reviews.

I'm pretty pleased with my butt myself.  I don't know that I would claim to have the "best", especially if "best" is defined as being muscular.  I take stairs and hike a lot, but I'm no gym rat (that sounds insulting which is not my intention... gym princess?), and my ass has got a great shape but is definitely not "hard as a rock".

If we still have photo theme days we could have an best-ass-themed photo contest on the NY board.  Would be one sexy thread!  Oh well.

I nominate Melaniee Benson. OMG love it.