Well, I am definitely a Microsoft/Windows user, so I will make a few comparisons about the newest versions...
Windows XP - When this operative system was launched for the first time, the Microsoft users experience something new and something different. The platform was very stable, system requirements was balances for the computers who was produced in that time of period and all programs and games was work perfect.
Windows Vista - This operative system was the biggest fail for Microsoft, that's why in short time period they launched the Windows 7.
Windows 7 - Was a mix of XP and Vista, but unlike them, this operative system was more fastest and his interface was quite better then Vista.
Windows 8 - Definitely the fastest operative system I was use. The way the Windows 8 turn on/of it makes me fell like his works with the speed of light.
Windows 8.1 - Is the same system like Windows 8, but some better improvements.
Windows 10 - I was thinking to download this summer, but I've decide to wait for the users review and also the new updates.