5 and 6 seems to be the core ones. 5 because it means that people can upgrade from kernel to kernel with an expectation that things will continue working (high

5 and 6 seems to be the core ones. 5 because it means that people can upgrade from kernel to kernel with an expectation that things will continue working (higher layers of the Linux "distro" stack would be wise to learn from that). 6 in large part because Torvalds specifically opted to not work for any of those major corporations back in the day, and thus can tell them to get lost when they try to get dumb things into the kernel (like Red Hat trying to get a Microsoft EFI key in there).