Interesting that busybox dc contains non-POSIX modexp operator | while OpenBSD's dc doesn't. Meanwhile, dc has been excluded from POSIX in favor of bc.

So, my top-5 of current POSIX standards is:

https://pubs.opengroup.org/onlinepubs/9699919799/utilities/awk.html

https://pubs.opengroup.org/onlinepubs/9699919799/utilities/ed.html

https://pubs.opengroup.org/onlinepubs/9699919799/utilities/bc.html

https://pubs.opengroup.org/onlinepubs/9699919799/utilities/od.html

https://pubs.opengroup.org/onlinepubs/9699919799/utilities/date.html