[manmal]

Get a low-end (HTC Legend), a mid-end (HTC Desire / Moto Droid), and a high-end (Samsung Galaxy Nexus or S-II/III) device, and you are good to go. Also get a cheap tablet if you need to support that. Always develop on the lowest end device as a rule, and the optimizations you apply to make it usable on it will make the app super snappy on higher end devices. Choose Android 2.2 as your target platform and you include 96% of Android devices: http://developer.android.com/about/dashboards/index.html

Always use dp instead of px, use dynamic layouts (like HTML/CSS) instead of absolute ones (like in iOS), use 9-patch or XML drawables wherever you can, don't fight the framework. Use the compatibility library for fragment support and other cool stuff, and use them wherever you can. Be careful about bitmap memory consumption, that's a gotcha on low-memory devices.

If you don't mess around with low-level stuff like NDK or OpenGL you should be good to go on the very most devices. The well-tread Java API paths are not as scary as some blogs want to make you believe. You don't have to own all the 10k devices which exist, because most of them have stock Android installed. For special cases you will receive stacktraces from people's devices in the Play dev console - or you use one of those crashlog services.

[trotsky]

highest density of good advice i've seen in a long time

[hayksaakian]

by moto droid do you mean the original moto droid? if so, i'd think by now the 2+ year old handset would be considered low end

[manmal]

Yes, the original one. No, HTC Legends are still around, those are the real low ends, also in terms of display resolution.