My favorite was the blue in Windows 2000. Never cared much for the green in Windows 95/98. When installing Windows 98 later (because games) I would try to set up the same blue. Instead of getting it correct, I guessed, and ended up with #336699 which was easy to remember and "close enough". I see now that it's closer to Haiku (almost perfect, #336698) than Windows 2000.

Same. The lighter gray 2000 uses compared to 9x is also much more pleasant.

It might sound ridiculous but using 9x with the default theme felt a bit depressing. It conjures images of being stuck in a building with that awful fluorescent overhead lighting on a day where it’s dark and dreary outside.

2000 on the other hand is about as refreshing as a gray 3D beveled OS theme can look, on par with Mac OS platinum and maybe slightly edged out by the bright colorful sunny tab titlebars of BeOS.