Longhorn will forever be one of the most interesting developmental periods of Windows GUI-wise. So many different themes were implemented but never used: Plex, Jade, Slate. Personally, I really love Plex's bold but simple design, keeping with XP's user friendly approach to UX.
Aero was the result of Longhorn: a fresh UX for the then-modern era that shows how far we've come in personal computing technology (to have literal windows
displayed on our desktops, for example). It was one of the last popular uses of skeuomorphic designs before minimalism took over the 2010s.
However, nothing will compare to the classic UI found on Windows 95/98. Sure, it's a bit clunky by today's standards (having so many menus and unnecessary options). However, it was easy to pick up and learn how to use a PC without much effort. Where do I
want to go today? When do I start?
(plus the icons were just really cool ok)