What type of ad is the most effective for monetizing my mobile app? banner, interstitial video? 

Depends on your games, you game metrics, your ad payment rates, how often you switch through ads, how many people actually click on the ads.  

Many games have turned to interstitial ads and also greatly increased the number of breaks in the design to allow many of them to be seen.  Players are more likely to click on them "accidentally" as they're full screen and unskippable and shown frequently, and more clicks mean more money.  

But even those are not highly profitable. Ads only work as a revenue source if there are an enormous number of people viewing the ads.