wrote this on July 26th 2015
The big two stores are obviously Apple and Android.
The graph below sums up the situation pretty well, with Android having 80% more downloads than Apple, while the situation is reversed in terms of app store revenue. Apple has approximately 60% more revenue than Android.
Amazon is the third store to look at, with revenues at approximately 50% of Android.*
Finally, Windows comes in 4th place, with revenue figures difficult to estimate, but reportedly much smaller than Amazon.
From our own experience, Apple typically accounts for 50-90% of total sales for our customers, with a fairly large variation coming with the type of content you're selling.
We are very close to releasing our apps on Amazon and Windows, so expect an update with our experience in these stores!