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I don't have much experience, but with what I've seen, Heyzap Mediation is the way to go. It's easy to set up and you will be covered with whatever you need. I see that LeadBolt is very bad because it simply can't compete with the others, Playheaven and Revmob are known to be bad so I didn't even set them up (Revmob is not an option in Heyzap anyway). Admob and Facebook for banner ads is a must. I have heard that Heyzap is very good for Android (and up to now I can confirm this). So if you go Mediation you have this set up automatically. Chartboost seems to be one of the best options for iOS.
I have been testing and playing around with this one for a little bit now. Im not an expert, but i had three games all getting around the same impressions and clicks. Three different ad networks. I have not been able to really make an opinion, as they all vary so much. One week one will be paying better then the others, the next week the others doing better. Im not 100% sure but i think it really depends on what the advertiser is paying to have their ads seen. As i said I'm not 100% sure. I have worked out that on weekends i seem to be getting more revenue than week days, with the same amount of impressions. Not sure but maybe some one more experienced can clarify things better.