1. Ganymede: The largest moon in our solar system, even bigger than the planet Mercury.

2. Mercury: The smallest planet in our solar system, but still larger than Pluto.

3. Pluto: A dwarf planet, much smaller than both Mercury and Ganymede.

4. The Moon: Earth's natural satellite, smaller than all three of the above.

So, the answer is Ganymede is the biggest, followed by Mercury, Pluto, and then the Moon.