For me it's macOS. Out of the three biggest desktop OSes it's still the most secure while being the most usable.
Since I'm also using an iPhone, it's a nice bonus to have some interoperability between devices, but most of it is turned off for hardening anyway.
Linux on aarch64 is a goddamn joke, sadly, and Asahi Linux has been stuck for quite some time in getting the rest of the features work on modern Macs. And I don't even want to talk about Linux security as a whole, which is still more than suboptimal.
Windows is definitely a no-go for me (personally scarred by tech support in help desk, from using it and just hearing what other sysadmins tell me, is enough to stay away from that pile of trash).
Besides, I'm getting a good bang for the buck on hardware. I regret buying my last Windows/Linux system, as it was overpriced for what it did and didn't really offer much security at all. That is one of the main issues: if you want good build quality on the PC side, you're paying more for the same specs, but things you don't even need (like extended warranties and on-site support).
Recent macOS developments have been a letdown too with the whole macOS 26 release, which is a bugfest. So I'm staying on Sequoia for now, until Apple can fix most of the bugs, probably somewhere in spring of next year.