First Ghosts, which easily trumps the other music tracks and is a really spooky song, especially towards the end.
Chauncey battle, which sounds disturbingly childish like a broken nursery theme.
Bogmire Battle is a very dark and foreboding theme for a dark and foreboding boss.
Boolossus battle is frantic. A given, since you're against a giant Boo that splits into smaller ones if you pop it. The other boss theme, Boolossus (split) is higher pitched and fast paced, with all the Boos that make it up ganging up on you as you frantically try to freeze and suck them up.
King Boo Battle, the final boss theme of the game, is darkly serious, even demonic as you fight King Boo and his Bowser mecha suit.
"Bros Before Ghosts" by Konrad Petersson for the fan album Harmony of Heroes. A jazzy, swinging remix of the Luigi Mansion's theme interspersed with sounds of Luigi traveling through the mansion and the ghosts hounding him.
"Boo Busters" is a delightfully spooky mashup/remix of the main theme and the Ghostbusters theme. If the iconic Ghostbusters lyics with Luigi's worried calls for Mario don't sell this enough, then the amazing mix of both songs definitely will.