If you have an existing app, use the Google Play Dev Console to find the phones your existing users are using and consider buying the top 2-3. These sites are

If you have an existing app, use the Google Play Dev Console to find the phones your existing users are using and consider buying the top 2-3. These sites are also helpful: http://choosedevice.com/ http://choosedevice.com/ http://www.handsetdetection.com/ http://www.handsetdetection.com/ My picks (with no context): Nexus 5 (straight from Google, use this as the gold standard), LG Optimus Fuel ($30 unlocked at Walmart, runs KitKat but has a tiny screen), Samsung Galaxy S4/5 (whichever is most popular), Samsung Galaxy S3 (still tons of those in the wild), and Kindle Fire 7" (covers tablet and FireOS).