Out of the four Galilean moons of Jupiter, only Callisto is at a safe distance from its parent planet that the radiation is low enough so one is able to colonize it. Io is totally uncolonizable (from a realistic point of view) and Europa too, however one could survive below Europa's surface if one drills through fast enough.
Now what about Ganymede? Ganymede's orbit is between Europa's and Callisto's, and from a realistic point of view, one could visit Ganymede for several days. However, is there any realistic way to be able to settle on Ganymede (or an orbit around it) permanently? Ganymede has an own magnetosphere which surely helps, and maybe on the far side of Ganymede there's less radiation than on the side facing Jupiter? How thick would walls have to be to shield you well enough from Jupiter's radiation on Ganymede's surface?