The most internally consistent design was that of NT 3.1 - it was a true classic in many respects. As far as general usefulness, performance, and versatility nothing can compare with Windows 10 (except Linux, of course).
Starting with Win8 you have basically a random choice if some setting is set in "classic style" or new fancy "settings" dialogs. You can have all the performance in the world if your users spend most of the time just looking for the right place to do something.
For example, the useless user settings in the control panel and the more useful old version in "control userpasswords2".
In Windows 10 you have two different kinds of UI and settings that can be changed in one but can't be changed in the other and vice versa.