Biggest relative to the planet it orbits. Total mass goes Ganymede -> Titan -> Callisto -> Io -> Earth's moon

Luna, our moon, is the 5th largest moon in the Solar System. Still quite impressive given the size of Earth compared to the Jovian planets. The largest four are Ganymede, Titan, Callisto, and Io. Both Ganymede and Titan are larger than the planet Mercury, which is quite interesting indeed, and Titan also has a dense atmosphere, the only moon with one of those! Callisto is a bit smaller and Mercury but still mightly large, and it has plenty of cool craters on its surface, Wikipedia says they are the oldest in the Solar System. Io is potato-coloured and is very volcanic, and everyone knows volcanoes are cool. Wikipedia says it's also the densest satellite in the Solar System with the strongest surface gravity!