I would vote for the Park Hyatt. It has the best view of the Opera House and in my opinion, has the best location. The service is excellent and the rooms and public spaces both are quite impressive. I also thought the food was excellent.

I would avoid the W (where I have also stayed) as it has a "druggie" sort of feel and the public areas are very dark.

I also hear the Four Seasons is quite stuffy and obviously, by its location, lacks a view unless you are on one of the top floors.