Windows 7. Some may say 8.1 if they were willing to tolerate it, but most will agree that Win7 was the last “true” OS that wasn’t riddled with adware and telemetry collection.

Microsoft lost a lot of money on Win8 (and by extension, 8.1). That made them rethink their business model, and they shifted away from selling the OS. Instead, they gave the OS away and made money on the data collection. Because Win8 made them realize that the world didn’t want or need a new OS every other year.

It’s the same reason people don’t upgrade their cell phones every year anymore. At first, the hardware changes were meaningful, and you actually got large upgrades with every new iteration. You were noticeably behind if you had a phone that was two or three years old. But now that modern hardware design has slowed down, (and hardware changes are more akin to updates on existing hardware), people don’t feel like they’re behind if they put off upgrading for two or three years.

And this reluctance to upgrade hardware meant people and businesses weren’t constantly buying a new OS every year. So Microsoft lost a lot of money when Win8 launched and everyone collectively went “actually, I’m good with my current computer.”