From what I understand:

-The intensity of radiation on Io and the surrounding space is practically unbearable for anyone, even with existing protections that can be added to a spacesuit without making it unusable. Although Io's volcanoes could be an attractive energy source once the other moons are colonized.

-Europe is an improvement over Io, though not vastly better. I think it can be endured for only a few minutes on the surface before the dose becomes lethal. Any attempt at colonization would have no choice but to be carried out by burying the habitats under the ice, preferably in caves or canyons.

-Ganymede's radiation situation is orders of magnitude better than that of Io and substantially better than that of Europa, plus the moon also has its own magnetosphere (albeit shadowed by Jupiter's magnetic field) such that human settlement on the surface would be feasible. It would also be a feasible candidate for terraforming.

-Callisto is the most promising of the Galiean moons for human colonization, due to its distance from Jupiter's radiation belts. It was also the target selected by the HOPE (Revolutionary Concepts for Human Exploration of the Outer Planets) study. Building settlements on its surface should be quite feasible, and it was even considered that such settlements could be used as a fuel producer for further exploration of the outer solar system.

https://en.wikipedia.org/wiki/Colonization_of_the_Solar_System

https://www.universetoday.com/130637/colonize-jupiters-moons/