* Brightest in reflected sunlight: Ganymede is the brightest of the four Galilean moons in terms of reflected sunlight. It has a high albedo (reflectivity) of about 0.43, meaning it reflects a significant amount of sunlight.

* Brightest as seen from Earth: Europa appears brightest to us because it is the closest of the four to Jupiter. While its albedo is lower than Ganymede's, its proximity compensates for this, making it appear brighter in our night sky.

So, the answer is Ganymede if you're talking about inherent brightness, but Europa if you're talking about how bright it appears to us from Earth.