Best: Google Play.
It often exceeds the expectations of developers. Google still constantly improves it.
Easy to submit updates.
Archive of all old packages.
Worst: Amazon AppStore
It may reject submissions due to their incompetent testers' errors.
The support is almost equivalent to zero. Their responses to support tickets often do not answer the questions or address any issues.
It may suddenly stop delivering an app to Amazon Devices, the primary source of app downloads, due to a policy violation. This can immediately drop the daily downloads from 3 figures to single digits. The policy may have only a general description such as:
"We consider a variety of factors as we evaluate each app, including the customer experience offered on the device and compatibility with the device. While our goal is to offer a broad and compelling assortment of apps to Amazon customers, not all apps available through Amazon Appstore are currently available on Amazon devices. Because your app overrides or alters the native user experience (e.g., with a lockscreen, widget, wallpaper, picture-in-picture or other on-screen element that impacts the native user interface), it has not been published on Fire TV devices. "
They do not tell developers exactly how an app violates the policy even if they are asked repeatedly to specify the violation for many times. It may take weeks for a developer to figure out the violation by trial and error, and lose a large amount of revenue. It may take a while to restore the daily downloads to the pre-damage level.
The best part of Amazon AppStore is the FREE app testing with results including screenshots.