That's a matter of taste, but for many it was the Windows they used first and/or spend the most time on and there's a lot of love for that reason alone. I never used XP all that much, but I always changed the theme to the Windows 2000 look. I really didn't like the default, it looked unprofessional and clunky in my eyes. Upon release XP was also pretty universally mocked as a Fisher Price-like UI.
To me Windows 95/NT 4 is still the gold standard in Windows UI. It's certainly not the prettiest, that would be Windows 2000, but it was easy to use, easy to navigate and efficient with space.
Took me years to like Windows XP. Hated the new start menu. I always kept installing Windows 2000 (or ME, probably because it could use Windows 98 drivers but look like Windows 2000).
I eventually got used to it and ended up liking it.
The first Windows I used was 98, and I sure didn't miss the blocky grey menus nor the pixelated icons. Maybe it was Fisher Price-ish but the new rounded buttons and borders were way more appealing to me.
The "Windows Classic" look on XP is exceptionally ugly, because those 3D-rendered icons just do not belong on a Windows 2000-style grey background for menus or buttons. (I have recently had to test both these OSes in VMs…)
win2k was peak for me. lean, functional, just a pinch of glitter here and there (short fade-ins). it was on par with the amazing stability brought by nt5 kernel.
i kinda miss xp at a cultural level since it was a bit the end of that computing culture cycle (after that apple started to dominate and ubuiquitous computing influenced desktop ui)