That would be 2000 or NT (I always confuse those two for some reason...).
All Windows branded as NT as well as 2000 (the first NT not branded as such) predate Windows 8 by over a decade, so I don't understand why you'd think one of them would be number 9.
Edit: Just realized GPs joke... There never was a Windows 9... And it isn't even that late...
Of course, that would leave out the consumer line starting with Win 95.