coding-system-priority-list

coding-system-priority-list is a built-in function in `C source code'.

(coding-system-priority-list &optional HIGHESTP)

Return a list of coding systems ordered by their priorities.
The list contains a subset of coding systems; i.e. coding systems
assigned to each coding category (see `coding-category-list').

HIGHESTP non-nil means just return the highest priority one.