Great challenge, Peter!
Firstly, I have to say that none of these asses compares to yours. I really mean it.
That said, I chose the most hairy one first followed by the submissive guy (great photos by the way), the shaved butt (is really asking for it), the skinny dude and the tattooed guy. I have to say I like tattoos, but that guy is not very attractive – too shaved, too white and too skinny for me :P.