Which isotope is used for a standard for all other atomic masses?

Carbon-12

Explanation:

Since the atoms are so small, it is impossible to measure their mass in a conventional way, but it is possible to determine relative atomic masses, that is, by comparing the mass of an atom of an element with the mass of another atom that is taken as a standard.

The most stable isotope of carbon, carbon-12, was arbitrarily chosen as the standard. Based on it, the atomic mass unit (uma) is defined

Then it is called atomic mass relative to the mass of an atom measured by comparison with that of carbon-12.

Your answer is carbon-12