It's a bit of a tricky question to answer directly, as "brightest" can be interpreted in a few ways.

Here's a breakdown:

* Largest Apparent Size: From Jupiter, Ganymede would appear the largest. It's the largest moon in our solar system, even bigger than the planet Mercury. This means it would take up the most space in the Jovian sky.

* Highest Overall Brightness: While Ganymede is big, Io is the brightest of Jupiter's moons due to its volcanic activity. It constantly spews out material, reflecting sunlight intensely.

* Relative Brightness Compared to Other Moons: This depends on the exact position of each moon. At certain times, Europa or Callisto might appear brighter than Io due to their relative positions and angles of reflection.

In short, there isn't one definitive answer. It depends on what you mean by "brightest."

It's worth noting that the moons of Jupiter would appear much more spectacular than our moon from Earth. They would be visible in the Jovian sky as bright, moving objects, even during the daytime.