I'd be tempted to like Android 4 but I think that's when they got rid of the framebuffer requirement for kernels so it meant you had to use Android for graphics

I'd be tempted to like Android 4 but I think that's when they got rid of the framebuffer requirement for kernels so it meant you had to use Android for graphics and couldn't just replace the userspace with eg debian anymore. EDIT: I've probably used up my quota so here's my reply: Userspace only cares about CPU arc. You can grab a debian rootfs and run it on a kindle/android/rasbpi etc. without even having to recompile things. The kernel however needs drivers and with modern Android many of them have to be rewritten. Replacing the kernel is a development exercise while replacing the userspace is mostly a matter of just installing the right software.