Re: The old ones are the good ones!
For a less Official and Jokey answer, Look to the build numbers:
Windows Vista: 6000
Windows 7: 7601
Windows 8.0: 9200
Windows 8.1: 9600
Windows 10: 10240
The real question is where did Windows 8 go, which is interesting on both a technical and philosophical level.
Supposedly the real reason there was no Windows 9 was for compatibility: lots of older/badly designed programs would assume Windows 9 = Windows 9x, and set themselves up for DOS instead of NT Windows - head that a lot, but don't know how true it is.