As this is a Moment of Awesome page, spoilers will be left unmarked. You Have Been Warned!
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!
All the treasures found in the haunted mansion determine the quality of Luigi's sweet new mansion. If you gather up over 100 million gold, you get the completed A Rank mansion, which is as big as the haunted one. It even gets fancy landscaping in the remake! Even more awesome is the remake-exclusive S Rank mansion. The mansion is even bigger, lavish home with pools and fountains.
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 tasks them with guarding keys to hinder Luigi's 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.
In the PAL version of the Hidden Mansion's version of Bolossus, Luigi starts riding on the Poltergust 3000, making movement fluid and smooth to more easily avoid the annoying Boos who attack you. It's not just efficient, but looks so cool you wish you could do it yourself in real life.
The S rank mansion, added in the 3DS remake. It looks less like a mansion and more like a high-class luxury resort!
, making movement fluid and smooth to more easily avoid the annoying Boos who attack you. It's not just efficient, but looks so cool you wish you could do it yourself in real life.