* We haven't seen every planet. We're limited to observing planets within our own solar system and a few thousand exoplanets (planets around other stars) that we've detected indirectly. There could be incredibly bright planets out there we haven't discovered yet.
* Brightness depends on multiple factors. A planet's brightness depends on its size, reflectivity (albedo), distance from its star, and the star's luminosity.
However, in our own solar system, Venus is the brightest planet as seen from Earth. This is due to its dense, reflective atmosphere, which reflects a lot of sunlight back to us.
Beyond our solar system, there are exoplanets that could potentially be brighter than Venus, but we don't have direct images of them to confirm.
It's important to note that when we talk about "brightest" planets, we're talking about how bright they appear to us from Earth. The actual brightness of a planet can vary greatly depending on our perspective.
