Which distro has the best GUI in your opinion?
Distro is irrelevant. DE/WM choice is all that matters as far as GUI goes. Also, if you want a GUI that looks or feels like windows then KDE probably has you covered in that you could probably customise it to mimic windows.
I quite like the Desktop Environment in elementaryOS. I think it's called Pantheon Desktop? It's very polished. Or InstantWM from InstantOS is also interesting and has some nice animations and effects.
Personally, I use simple and minimal Openbox
