Honestly, there is no "best" ad network. Why? Well because each game is different.
Depending on the app's design some ad formats may work, some others not. And depending on the geo, each format brings different results.
Also, for some ad networks, there is a minimum amount of impressions needed for them to "activate" and work, let's say 10k. So for developers who are starting that's something to keep in mind.
The best you can do is:
1. Think carefully about how you want to monetize and think about a strategy that includes different ad formats (banners + videos at the end of each session).
2. Use mediation so you can get the highest paying bid for each impression. Mediation integrates several ad networks at the same time and makes them compete in real time.
3. Make sure your mediation platform displays metrics, and keep an eye on them to make adjustments in you in-app ads.
Why don't you try Appodeal? They have this performance index you can check for more insights on the performance of different ad formats and regions.