According to NASA, Jupiter's moon, Ganymede, is the largest moon in the solar system. It has a diameter of 2,631 kilometers. Ganymede is larger than both Pluto and Mercury and is 3/4 the size of Mars. Unlike many other moons, Ganymede has a core that generates a substantial magnetic field.
Charon, the largest moon of Pluto, is the largest moon in the solar system relative to the world it orbits. It has a diameter of 1,207 kilometers, just over half the diameter of Pluto. Charon is also tidally locked to Pluto. As a result, it always shows the same face to Pluto.