The Westin, the Park Hyatt and the Crown Towers are the three top hotels in Melbourne.
My favourite is the Westin for its location in the centre of the city. It is also quite luxurious.
The Park Hyatt is close to the CBD and is quite opulent, but has historically had service issues. But on a good day it can be very good.
The Crown Towers is probably the most "popular" luxury hotel in Melbourne and is probably the most glitzy (in a casino-ish type way) option in Melbourne.
The Langham is an older hotel - it opened in 1991. It should be fine if you are not looking for anything that special, but is not really a top tier option.