What is the strongest of the four fundamental forces?

The Strongest of the above listed fundamental forces is the Strong Nuclear Force which exists between Nucleons.

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The weak nuclear force isn't a force in the conventional sense. It is a force in that it is mediated by the W and Z bosons. The weak force is responsible for transforming quarks from one form to another. This is usually manifested as beta decay where a proton is converted into a neutron, a positron and an electron neutrino, or a neutron is converted into a proton an electron and an electron antineutrino.

The residual strong nuclear force is actually a residual effect of the colour force which binds quarks into baryons or mesons. The strong force binds adjacent protons and neutrons to form atomic nuclei. At the scale of the nucleons protons and neutrons, the strong nuclear force is by far the strongest.

The electromagnetic force is responsible for the interactions between charged particles. It has a much longer range than the strong nuclear force. In the case of elements heavier than lead, the electromagnetic force causes every proton in the nucleus to repel every other proton. At this scale the electromagnetic force is the strongest which is why heavy elements are radioactive.

Gravity is not actually a force but is the results of the curvature of spacetime caused by mass. It does however behave like a force in the Newtonian sense for objects which are not too massive and are travelling much slower than light. Gravity is insignificant at the atomic scale where other forces dominate. However at the scale of planets, solar systems, galaxies and galactic clusters, gravity is by far the strongest force.