Anything amateur or homemade where it is genuine and you can clearly see that the woman is really enjoying herself. In these circumstances, the more extreme, the better... the same is also true for dog porn (although finding content like this is very rare).
Usually I gravitate toward homemade or authentic experiences like strangers or amateur couples doing things inside or in public, flashing, Webcam stuff, rough sex and anal, and almost anything reverse cowgirl.