1. Ganymede: The largest moon in the solar system, it's also the brightest of the four Galilean moons.
2. Callisto: Despite being slightly smaller than Ganymede, Callisto is slightly dimmer.
3. Io: The most volcanically active body in the solar system, Io reflects less light due to its sulfurous surface.
4. Europa: The smallest of the four, Europa is also the dimmest.
It's important to note that the apparent brightness can vary slightly depending on their position relative to the Sun and Earth.
