I don't think MSN AdCenter has any publisher accounts available yet. Yahoo perform best for some people, especially for US/UK traffic. If you're having international traffic, I'd go with Yahoo provided that you live in the US/UK (as they only allow publishers of those countries to signup).
I think ads by goole is the best one which is most familier in the world. Again people are aware from this site also. Google has never cheated to any person since today.