Well this is a very open ended question, so for me since I make money selling porn, its gotta have something unique be it content, presentation or whatever. kink.com has unique content that isn't on all the tube sites and converts well. Videosz.com has a ridiculous amount of content people can sign up for less that $5 and download dvd after dvd of the stuff without any worries they will get a virus and on the affiliate side you can sort it by pornstar, which is perfect for me. And their sister site ztod/3rd/blackice has TONS of HD content.
But the best is the most unique program I promote because so few people promote it, and its a one of a kind site. And no one competes with me for the keyword.