Since Play Store is the largest, I'd go for N7. but develop against Gingerbread, ICS and JellyBean (All versions of each) It's just a matter of choosing which to compile against in the SDK. Start with Gingerbread (Basic version) when that's running properly compile against ICS and add ICD specific features if you want. repeat with JB. That way you have the widest possible audience in the Android community, and at least one of those versions should work on the other devices.
Have a look at wnat Amazon have to say about developing for the KF and what Nook have to say about their App store's requirements.
Take you rime and do the researh forst, it makes life so much easier in the long run.