Ballmer's tenure started with XP and eventually gave us Windows 7. Nadella gave us 10 and 11. Though 10 was largely developed under Ballmer before its initial launch, it's been under Nadella's stewardship ever since. I'll take Ballmer, thanks.
Also, Vista was a mess, but it was ambitious and it laid the foundation for 7 and it was only around for a few years before being replaced. Unlike 10, where Nadella has doubled down on the worst choices and even worse ones were made going into 11 (HOW THE FUCK DO YOU SET DEFAULT APPLICATIONS NOW, THIS IS FUCKING NONSENSE).