The simple answer is that we have over a thousand years of jurisprudence to answer that question. In some cases it's the next of kin; in some cases it's a state-appointed guardian.

But even with the benefit of all that law we still have many courtroom arguments over who gets to decide what is best for individuals who are only partially able to speak for themselves. When we move beyond natural persons and corporations, we have far less jurisprudence to rely upon and they can't express their desires at all, so it's harder on both counts.