Windows. Linux as a desktop just isn't stable enough for me. Too many bugs with GUI settings. I don't want to look up multiple ways to do things just to be eventually kicked into the command line and hopefully run the right commands to get some basic settings working. I'd love for Linux to be more stable and to have a cohesive GUI.
On the more cosmetic side running KDE apps in Gnome or running Gnome apps in KDE is just a further huge mess that can essentially ruin how your system looks which could potentially soft-lock you on screens that you can't read. The DE on Linux just should do the Windows and Mac thing of requiring hooks to allow them to set important color and theme settings.
Windows is terrible but it's still leagues above Linux in some real basic ways. Linux is going to need to step it up if it ever wants a serious "year of the Linux desktop" to happen before the death of the desktop computer altogether.