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. He manages to trick Mario into entering the mansion ("Seriously, though, who would actually believe that mansions get given away in contests?!") and captures him. He hides along with the rest of the Boos to ambush Luigi, blows him away from the altar where Mario's painting is (if Luigi hasn't captured 40 Boos), frees the dangerous Portrait Ghosts and locks away parts of the mansion so Luigi can't progress. It's entirely possible the lightning strike that caused the blackout was his doing. Furthermore, despite his fear of the Poltergust 3000, he's courageous enough to get over it and face Luigi in battle albeit with a Bowser suit. That takes some doing.