Microsoft's best OS of all time was Windows 2000. Imho, Snow Leopard was Apple's best. There is something nice about a bug free operating system that "just works" as intended. No problems, no hassles. No load times, even back then. Back then it was common to have a computer with 128MB+ of ram but Windows 2000 ran on 64MB of that, so it was the first non page file heavy OS that just snapped. In comparison, a year late…
No load times?! I seem to remember Win 95-XP era involved almost comically long boot times especially if you waited for your hard drive to stop churning after getting to the desktop, but maybe I am misremembering. It wasn't so much the OS's fault, though.
But I've compared how long it takes to start Word on my Pentium II 450 MHz running Windows 98, with Word 2000, to my Core i5 2500 K (3.3 GHz) running Windows 8.1, with Word 2010. The Pentium II wins.
I don't how much of the difference was Windows versus Office, but it's a sad indication of the slowdown in software outpacing the speedup in hardware.