IMHO, "the best" is not suitable for all publishers; it depends on the traffic source and the nature of your websites. The most important thing is the efficiency of the PPC. I worked both with the AdSense and AdCenter, and I have to say that they have a lot of traffic, however, the but the efficiency is not that exciting. I decided for myself to start trying working with the smaller PPC specialized in targeted advertising. I must admit that they are more efficient. In addition, they pay more often, which is also a very pleasant thing and a big plus on their account.