Already, with only three posts, iy comes out as a matter of personal opinion and, possibly, experiences - good or bad.
For me, Windows 8.1 is the best and slickest OS they have produced to date. I did customise the modern start menu and find it more convenient than a text menu. But, having said that, I see that particular function as having been over emphasised and over discussed. A simple third party menu, of which there are many, sorted out that problem. Other than that, my 8.1 installations have now run for some years without any hitches.
As an OS only, Windows 10 is, on my old equipment, noticeably faster all round. The two major additions, Cortana and Edge, have no interest for me, in spite of over advertising and pushing from Microsoft. But, for the moment, apart from the overall improvement in speed of general operation, I have not found anything in Windows 10, which I cannot accomplish in Windows 8.1.
No comment, unfortunately , on Windows 7. I have not personally used it for some years and in my daily rounds, do not come , too often, in contact with anyone using it.