The smallest subatomic particle is generally considered to be the quark.
Quarks are fundamental constituents of matter that combine to form protons and neutrons, which in turn make up the atomic nucleus. There are six types of quarks—up, down, charm, strange, top, and bottom—and they are never found in isolation due to a phenomenon known as confinement. Instead, quarks combine in groups to form composite particles, such as protons (made up of two up quarks and one down quark) and neutrons (made up of one up quark and two down quarks).
Unlike other subatomic particles such as electrons, which are recognized as fundamental, quarks have not been observed individually in experiments. This indicates that they are truly elementary particles in the Standard Model of particle physics, which describes the fundamental forces and particles in the universe.