Desktop
With the awful W8 or 8.1 user interface modified by Classic Shell to make it look like W7, W8 is fine for this low power mouse and keyboard user and is much faster than Windows 7. I can't imagine what new features W10 could would bring for me except for a better back up system to a network attached drive.
Like a lot of commenters here, I have not enjoyed the Android experience on an HTC Desire X from O2 and I am seriously considering joining my wife on MS/Nokia windows phone if I can find one with two cameras and a flash.
As for some of the very useful Android apps, a budget tablet with a mid sized screen will probably do the job, mainly to run fancy astronomy simulations.
And for the very little and simple computer work I do, there is always Ubuntu running flawlessly on a 7 year old AMD desktop.