I've got Windows 7 Pro with SP1 and I am even happy of it but I tried Windows 8 it was terrible. I thinking about buying upgrade to Windows 10 but will it be better?...
My question is: Which version of Windows is the most stable nowadays?
Windows 8 was never great out of the box, but the update to 8.1 made it so much better. Windows 10 doesn't seem that much more different from 8.1, but then again I hardly use it.
For the sake of longevity, you'd be better off going to Windows 10. It's the most recent OS, there's going to be a lot of development both on it and for it, and will have the most support going forward.
Though it makes me ask the question, why is it you must use Windows? There are other operating systems out there which might meet your needs just as well, or better.