The most concentrated form of energy is nuclear energy, specifically the energy released during nuclear fission or fusion. In nuclear fission, the nucleus of an atom, like uranium-235 or plutonium-239, splits into smaller parts, releasing a tremendous amount of energy. In nuclear fusion, which powers the sun and other stars, light atomic nuclei combine to form a heavier nucleus, also releasing vast amounts of energy.
To put this into perspective, the energy released from just a tiny amount of nuclear fuel can power a city for an extended period. Unlike fossil fuels, which release energy through chemical reactions and have a lower energy density, nuclear reactions release energy due to changes in the nucleus of an atom, making them far more powerful. This high energy density is why nuclear energy is considered the most concentrated form of energy available today.