Hey!
From my experience, Admob delivered the highest eCPM (up to $6 from interestitials). Of course, it depends greatly on where you place banners and how much traffic you provide. networks like Millenial media fail to deliver ads when traffic volumes is low.
I also saw great results from Airpush (but be carefull with spaming users with push notifications, theyll see it
s coming from your app). You can set the frequency of pushes and their type.
Lastly, I now work for GoWide company, and can recommend it as well (it`s up to you whether to follow the link