So, which Windows version is best for QW nowadays? I remember trying WIn10 when it was new, but I got lower fps than with Win7 and went back to the latter. Could be that I didn't turn off some Win10 stuff that was slowing things down.

Most new hardware won't support Windows 7 these days... Both my latest Desktop and Laptop only have Windows 8.1 and Windows 10 drivers available. Windows 10 is fine to use though so its no big problem.