Following up on the above, surveys and offerwalls will give you the highest return, though they don't work with every app. I'd highly recommend Rewarded video which would be the next highest paying format after survey/offerwall. They pay high rates and are very non-intrusive. Incentivising users to engage and earn in-game currency/unlock new features. You can probably get away with added skippable video/static interstitial inbetween levels or when the user loses a level. Banners are okay, and good if you have a lot of users but I personally find them annoying as they just stick to the screen. Its like you're trying to watch tv and theres a slight crack or damaged pixels on the screen that you try to ignore. Native ads look really cool. Perform better than banners and have the same look and feel of an apps content. Also very user-friendly and non-intrusive.
very right! often the urge to complete, publish, hope for an impressive ranking and passion in general can make devs to do these wrong steps.