Luigi, for arguably the whole game. Think about it. He is entering a mansion filled to the brim with ghosts. He's only armed with a modified ghost catcher, and is clearly frightened out of his wits. Why does he do all this? So he can find his brother. In the end he manages to capture all the portrait ghosts, and save Mario. And how long did it take him to do all this? Just one night. He even gets his own not-haunted mansion!
King Boo is smart, especially compared to other Mario villains.