Best ad network for banner ads

Can you guys please suggest me the best ad network for banner ads? I have used startapp, admob and i am currently using airpush. Airpush is giving me very low fill rate and very very low ecpm. Their sdk also requires many permissions and is also very buggy. Their ppd model is also not so good.

You can try appbrain. You can even customize your banner in it and the banner will show without internet connection too although that will not be counted as impression.