There are actually four planets in the Gliese 581 system, but I'll assume you mean Gliese 581b as it is nearest Neptune's mass. Though the planet or star doesn't really matter at all to the story.
I'll say that they're all equally smart.
-The fish has, as aika said, absolutely knowledge of everything on the planet at every instant.
-The man has the knowledge of what and how he wants his environment and the ability and time to transform the environment into that state.
-The civilization had developed the basic ability to influence its environment, and is continuing to learn and grow.
However, each occupies a distinct niche on the planet, and, save for the fish observing, do not influence each other in the slightest. Therefore, each is the smartest possible in its own niche.
You could probably also argue that the fish is the smartest, as it is the only one to have knowledge of any of the others, but I won't.
