* Academically: There's no universally agreed-upon highest academic degree. While a PhD is often considered the highest *research* degree, there are professional doctorates (e.g., MD, JD, EdD) that are terminal degrees in their respective fields and not considered "lower" than a PhD. Some institutions even offer further postdoctoral degrees or fellowships.
* In a specific field: The "highest" degree would be the terminal degree in that field. For example, an MD is the highest degree in medicine, a JD is the highest in law, and a PhD is the highest in many academic disciplines.
So, there's no single answer. The highest degree is context-dependent.