The Electron Cloud orbiting the Nucleus.
(The question is about Volume NOT mass).
the region with with highest probablity of find electrons.
the "not the nucleus" region of an atom
let's take an example.. carbon-12.. 6 protons.. 6 neutrons...
approximate volume of the nucleus = 12 x (4/3) x (0.5x10^-15m)3 = 2.0x10-45 m3
approximate volume of a carbon-12 atom = 4/3 x (85pm)3 x (1m / 10^12pm)3 = 8.2x10^-31 m3
volume fraction in nucleus = 2.0x10^-45 / 8.2x10^-31 = 3x10^-15..
ie... approximately 0.3 trillionth of a % of the volume is in the nucleus. the rest of the volume of the atom is in the "not the nucleus" part of an atom.
and what the "not the nucleus" is is technically a region in space with higher average electron density (than the nucleus).
the "electron oribiting a nucleus" model aka the "Bohr atomic model" became obsolete when quantum mechanics came along in the 1920's.
You use the unit "amu" (Atomic Mass Unit) to describe the mass of an atom or parts of an atom.
Electrons are approximately 1/2000 amu, so they nor the electron cloud are the "heaviest".
Protons and neutrons are each 1 amu, and since they constitute the nucleus, the nucleus is the "heaviest".
Update: sorry about the misunderstanding. I've just never heard anyone describe an atom's volume.
The orbiting electrons of the atom will make up most of its volume.