The largest moon in our solar system is Ganymede, which orbits Jupiter. Ganymede is even larger than the planet Mercury. It is one of Jupiter's four largest moons and was discovered by Galileo Galilei in 1610.
As for the moon orbiting Neptune, it is called Triton. Triton is Neptune's largest moon and the seventh-largest moon in the solar system. However, unlike Ganymede, Triton does not have a retrograde orbit. Triton has a retrograde, or backward, orbit, meaning it orbits Neptune in the opposite direction of the planet's rotation. This is an unusual characteristic among the major moons in our solar system.