I know this sounds petty but the XP’s Classic’s task bar gray was ever slightly off from Windows 2000 and it drove me nuts. I think Windows 2000 was visually the best.
Once you were back to the 9x/2k style chrome, I'm fairly sure you also had access to the older style of color/size customization for it as well. On a minor tangent, I miss how well the high contrast theme used to display on windows itself and the majority of applications that were still made within that standard toolkit, using it now seems to lack polish and most applications are doing their own thing within their window.
