Well here are some off of the top of my head that I recall.
1997 - Hackers exploited a bug in Microsoft's(MSFT) server software, jamming the company's Web site.
2000 - Software giant Microsoft is trying to play down the damage done after malicious hackers infiltrated its computer network.
BBC News | BUSINESS | Microsoft downplays hack attack
2001 - Microsoft became the prominent victim of a hack that attacks the domain name server. In these denial-of-service attacks, the DNS paths that take users to Microsoft's Web sites are corrupted.
2012 - Hacker group, Swagg Security released a massive amount of data including email logins, server logins, and bank account credentials of large companies like Apple and Microsoft.
2012 - A lot of important Turkish Websites are hacked by F0RTYS3V3N (Turkish Hacker). Microsoft’s website along with other major companies was hacked as well.
2013 - Microsoft didn't tell the public one of its databases was hacked in 2013.
Microsoft didn't tell the public one of its databases was hacked in 2013
2019 - Microsoft reveals certain Outlook.com user accounts were hacked for months
Microsoft reveals certain Outlook.com user accounts were hacked for months - Security Boulevard
If you count all GNU/Linux distributions as one OS, I’ll have to say GNU/Linux with Ubuntu leading the pack.