XP was a disaster, security-wise. Vista was better conceptually but the tech was undercooked and win7 was basically a re-skin of vista plus some extra maturity due to time.

Then, there's the entirety of the situation around those releases. Microsoft was so mismanaged under Ballmer that even though Windows 7 was maybe their best OS, it completely missed the smartphone revolution and set them back about a decade until they could be re-spun as a cloud technologies company.

Imagine how huge they'd be if they had successfully leveraged their desktop and business dominance to force their way on top of the mobile world.

Tl:dr Win7 was both Microsoft's best and worst product of all time.