Ganymede

Ganymede is the biggest of the Jovian moons, as well as the biggest moon in the solar system, bigger than Mercury and almost as as big as Mars, though not that massive. It also has a magnetic field, which can interact with Jupiter's magnetic field in some weird ways. Ganymede could hold an abundance of water, and could have more water than all the oceans on Earth. Ganymede even has a small atmosphere, which contains some oxygen.