"Peak Windows GUI" and "Peak GUI" are two separate things.

For "Peak Windows GUI", MW10 and MW11 both score high in my opinion, but the changes in Start Menu behaviour in MW11 and the horrible "Show more options" sub-menu in the MW11 right-click context menu are confusing. So I'll give MW10 the advantage for consistency and less insult to the principle of Least Surprise.

For Peak GUI, I would say there's a tie. An Android device with Desktop Mode is just hard to beat for multi-context usability. Early OS X looked great and had mature GUI ideas. And my daily Linux box with the Sway tiling window manager is the right combination of mouse gestures and keyboard power.