A serious android developer should have more than one phone. Ideally, you should have at least one nexus-like device to test everything. Since the OS is unaltered in the nexii, it’s easy to locate the source of any problems. Then, you should get some extra devices, with more or less ram, different screen size, different brands (go for popular vendors in that case). In general terms:
If you need the phone purely for testing, you could as well go for Genymotion and test on so many devices with different screen sizes and all that.
If you want a ‘new, classy phone', you may try the google pixel. Its from google. Google officially supports android. Google did the hardware. Google did the software. There are nice reviews for the pixel on the internet. You have extra bucks to spend? You might as well go for the Google Pixel
-Keep a low range device (anything with ICS, the minimum supported version now).
-A mid range device (an old flagship or nexus 4 or 5) . You should do the bulk of the testing here.
-A high specs device (a nexus or pixel). Ideally, this one should be in the latest android version possible.
