I'm still fairly new at this, but Chartboost is the best choice from what I've experienced. Admob won't add up to much unless you're killing it in downloads. I'm going to start focusing on closely targeted ads that work with my gamers. I can still make some coin without bombarding them with ads.
On a side note: with admob. I have theory that you seem to get a much higher ecpm when you put the banner ad at the bottom of the screen, rather than at the top. I am not 100% sure on this, but I do think it has some influence. I have two very similar apps, one has a banner at the top, one at the bottom. Both are photo apps, and both only have a banner ad: Ad at the top: ecpm $0.74 - 38,606 impressions last 7 days https://itunes.apple.com/us/app/easy-eraser-remove-unwanted/id839750800?ls=1&mt=8 Ad at the bottom: ecpm $3.43 - 24,724 impressions last 7 days https://itunes.apple.com/us/app/magic-instasize-no-cropping/id883971274?ls=1&mt=8 There could be a number of reasons for this, I will be doing an update soon and moving the banner to the bottom on the first one. This will give me a much better idea if this is what causes such a big difference.
That's interesting. Are they universal apps or iPhone/iPad specific? If you did a case study on me (probably not the best subject), I'd say I completely ignore top banners on my phone, but I'm likely to touch a few on my iPad.