windows 8 is a strong candidate, because that was their huge push into trying to remodel the OS in the image of mobile OSes. you had to perform quite the exorcism to get it functional. i skipped Vista so I'm not the best source on this but my understanding is that the issue with Vista is less that it was loaded with dark patterns and trying to be a walled garden and more just an unfortunate time to be an OS with the technology and security landscapes changing.

of course, while the base OS wasn't necessarily always the problem, Microsoft has anti-competitive practices going back even further and you could argue Windows stopped being good when MS started bundling Internet Explorer with it, so it all depends where you draw the line. might be safest to say their last truly good OS was MSX-DOS just because they abandoned it before they could do anything scummy