I am not sure what your definition of a monster is, but I think several people agree that a Gold Saum (Green Terror) is an awsome fish for a 55. It would be a good beginner fish too. A full grown male is a stunning fish. They are easily available; you can get them at Petsmart if not at another LFS.
Check them out here:
http://www.monsterfishkeepers.com/forums/showthread.php?t=55682
Check them out here:
http://www.monsterfishkeepers.com/forums/showthread.php?t=55682