What version of MacOS X is the most stablest for older Macs?
What of all the MacOS software versions is the most stable? Is my version the most stable for my MacBook? The MacBook is a late 2009 13 inch on Sierra.
Stable? High Sierra should be fine provided you've got enough RAM and a fast enough drive.
Of course, High Sierra is as far as it can go, and that is likely to drop off the list of supported OS's altogether within a few weeks when Big Sur is released.
Honestly, I'd be seriously considering replacing that machine. It's had a good life. It's time to move on.