in the starwood family, the westin is probably the place to stay, lounge is nice, bathrooms are excellent, rooms are ok to above average. the gym is excellent and you get free access if you're on the exec floor or a platinum.

there are other properties not part of the spg family that are also v.good in sydney, as mentioned the observatory hotel, park hyatt, four seasons. the westin has more of a business hotel feel for my liking. the four points is excellent value for money and in a great location if you get an A$149 rate by calling the hotel directly (some nights are excluded though).

for canberra, stay at the hyatt and the rates are fair, you can also try http://www.wotif.com which i think often has the hyatt in canberra listed.

melbourne - stay at the westin, it's a popular hotel or for boutique from what i've read folks like the como.