What contributes the most to an atom's mass?

An atom's nucleus is made up of protons and neutrons. Of all the subatomic particles, it is these nucleons that are the most massive, and as such, they contribute most to the mass of the atom. The measure of an atom's mass is defined by the amount of protons and neutrons in it's nucleus, the neutron being about the same mass as a proton, which is considered 1 atomic mass unit (1 AMU).