Ganymede is the largest moon in the solar system. It is possible to colonize and terraform Ganymede. Ganymede has salt water ice. But probably by the near future, people would probably remove salt before it could be drinkable. Ganymede could have a subsurface ocean. If there is life, it is possible but not certain that colonists would have to avoid it. Ganymede is not in the radiation belt. So it could be colonized on the surface. Ganymede may be terraformed by using extreme greenhouse gases in the outer solar system. There is, however, a risk that the vast amounts of water ice present may turn Ganymede into an ocean world, but the excess water could be moved in the form of ice and used in the terraforming of Venus. The resulting mix of land and ocean would greate a hydrological cycle with rain and freshwater lakes on land. It is possible that Callisto may not have that problem.
Nederlands; Kolonisatie van Ganymedes