Chartboost. Used minimum floors. Chartboost only recently started implementing corona sdk support, where as before you had to be enterprise Corona (I know some people who are, and they are really happy about this as they don’t have to pay enterprise prices anymore).
My only concern is you have to be careful where you show the ads, since they are full screen only. You can do it on bootup, and on game over. I don’t recommend on retry as the ad sometimes lags so the player may hit retry, start playing and the ad shows up. This happened to me, and I submitted an update to address that issue. The lag is due in part to network conditions (3g, vs 4g and the quality of connection). If the speed is slow, then the ad pops up later. Keep in mind, you set up a cacheInterstatial() (which pre loads some in advance) but once those run out then it’s pulling in real time. I could blab on an on about that, but I think you get the idea
Check out their site, they have a github with the corona plug in. The analytics are great.
Let me know if you have more questions, it’s pretty interesting
-Nick