I liked Windows 2000 too... only reason I upgraded (at the time) was because it lacked support for drives bigger than ~135GB.
Overall though, my favorites:
Windows 7
Windows XP
Windows 2000
Windows 3.11
Least favorites:
Windows Vista
Windows ME
Windows 8
On a related note, Microsoft's always been comically bad with their product numbering schemes...Windows 1, 2, 3, NT, 95, 98, 2000, ME, XP, Vista, 7, 8, 10... calling the 3rd Xbox an "Xbox One", etc.
Life is like a box of chocolates, it doesn't last as long for fat people.