Pimproll is my favorite sponsor for numerous reasons. They convert like mad for me...they rock when it comes to variety of sites, especially their ebony niche.
They pay on time ALL the time! Thier webmaster support simply rocks. Phil bends over backwards to answer any questions you have and tries to accomodate any special needs you have.
If you notice a problem they are on it right away. They pay well at $35 - $40 bucks a sign-up, they have a ton of hosted galleries and the list of what I like about them is endless. It doesn't matter if you send one sign-up a week or 100 they treat you like you are valued and important. They are not the type of sponsor that looks down on you unless you send them massive amounts of sign-ups. They understand that even smaller webmasters become bigger ones and they are right there to help you achieve your goals.