That is a tricky question, which astronomers didn’t answer correctly until fairly recently.
For a while, they thought the biggest moon was Titan, which orbits Saturn. But a deep atmosphere topped by orange smog surrounds Titan, so scientists couldn’t see its surface. Not until spacecraft began to visit Titan at close range could they measure its true size and find that it’s only the runner-up. The prize for biggest moon goes to (the envelope, please) … Ganymede, a moon of Jupiter, at a diameter of 3,270 miles (5,262 km) — about 70 miles larger than Titan.