Advertising Networks

Hello everyone. I have looked around and found out from different people that there isnt a best ad network and that more than one must to used in your app. For what I hear aswell its done through mediation. I was wondering how you choose your advertising networks. Do you look at who offers the highest ecpm? Or do you base it on something else? Also could I have some examples of some good networks to mediate so I can get started with all this? What is the strategy that you use? Thanks.