Mobile ads for publishers (i.e. mobile app or website developers)

Nowadays, there are many (too many, IMHO) advertising networks (Google's AdMob, Apple's iAd, Microsoft's pubCenter, etc.) for mobile developers to monetize their apps. 

There are a few parameters considered when choosing an advertising network. The most important one is eCPM (effective Cost Per thousand iMpressions). Unfortunately, eCPM varies dramatically (often by one or two orders) from time to time, between networks and between apps.

New advertising networks usually provide high eCPM's at the beginning, but they drop over time. 

Good user experience is another important factor. Unfortunately, intrusive ads usually pay more but annoy users. 

In conclusion, make sure the type of ad does not make the user uncomfortable, then try different networks and watch their performance closely for choosing the most rewarding one.

Amazon Mobile Ads has poor SDK causing all kinds of ANRs and crashes. It seems that they are not very serious about it because some of their document pages are "404 Page not found".  Their fill rate can be close to zero.  However, Amazon offers the highest ECPM as of January 2020.  Fill rate: 3.65%, eCPM: $1.15. 

Amazon phased out Amazon Mobile Ads in July 2021 and replaced with Amazon Publisher Services.  It is so casual on this that their SDK does not even have a repository.  One has to download their SDK. It is essentially a middleman among middlemen.  One has to sign with other mobile ad services to use APS. The uncertain benefits are not worth the hassle and risk.

AdMob from Google has the worst customer service or no customer service. One cannot change an account from an individual type to a business type.  One cannot create a new account of a different type after closing one.  It once had high eCPM, but has the lowest eCPM as of January 2020.  Fill rate: 14.77%, eCPM: $0.07

Epom may sound good, but their technical support does not exist.  If one sends a technical question, they don't bother to reply though they respond to non-technical questions.  Using Epom could potentially a disaster - nothing works after wasting many hours on it. 

 InMobi is good until it deactivates your account and their customer service becomes unresponsive.