What Android phone should an Android developer buy in 2016?

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.

Why don't smart phones run different operating systems, like Android alongside Linux and Ubuntu

The computer hardware follows a “computer architecture”. Each architecture has an instruction set, “assembly language” that contains basic commands like “add, multiply, store, load, …” that can be executed by the hardware. The operating system is the bridge between software and the instruction set of the architecture. Nowadays most pc’s use x86 64-bit architecture. If you have a 32 bit or AMD64 architecture, you’ll need an adapted version of Windows or Linux.

Special snowflake Apple likes to keep everything in their own hands, so they made their own architecture to work with Intel processors (often called “Mactel”), and MacOS is only available for that architecture. There are some third parties offering MacOS-like systems for x86 and vice versa, but those are definitely not optimal. It’s take the whole package or take nothing at all.

For Apple’s iOS it’s the same story, they own the architecture, hardware and OS, and if you want a different OS you’ll have to sacrifice some efficiency. Android is based on the Linux kernel, and is in theory modify-able to your needs. Ubuntu Touch for example is an Ubuntu system for smartphones and tablets. Given the limited possibilities compared to PC’s and all incopatibility issues, there is simply no reason to do so. There is no real market for different OS.