I think it depends whether you want to include Windows 2000 in the discussion. It was solid and I used it until XP came along but it was built on NT and marketed as more of a server option. I don't remember it ever being included by OEMs. I think most people went from 98SE (or god forbid, ME) to XP. I think its popularity and driver support probably led to XP but in a canonical list of MS consumer operating systems I would probably skip 2000.