Callisto is Jupiter's second largest moon and orbits the furthest away from Jupiter. Unlike the previous 3 moons, it is not in any resonance with the rest of them. The terrain is not veryeven and can be slightly difficult for inexpirenced players.
Callisto can be considered to be the easiest moon of Jupiter to go to, as the distance from it's host planet makes you go lot slower than closer orbits like near Io.