10.5 Leopard was my peak. 10.6 Snow Leopard improved on its stability but had a touch of visible regretion (in my opinion). After that, I haven't really been happy with any release. Obviously some things have improved but... 10.5 was branded Unix, the UI was beautiful. It felt solid.
Put Linux on the thing? I'm using an ancient MB Air with Debian, works just fine. Same for an ancient 27" iMac, also running Debian without any problems.
I switched from Linux to macOS but the lack of QA is going to force me back to Linux once security updates stop for Sequoia.