Google Adsense is currently the most popular and easy to get into. YPN pays better, but has terrible ads and is hard to get into. The lower tier CPC networks are: Adbrite, Clicksor, Bidvertiser, etc. But you should only apply to those if you can't get into Adsense or YPN.
Of course Google Adsense is the best.YPN seems better on the CPM rate.But it's hard to get an invite even you are in the U.S