The Park Hyatt is just a little isolated in terms of location. Not bad mind you, and surrounded by gardens which is nice, just a bit more walking for most of the major sights. The Sofitel is in a better location, but I still give the edge in location to the Marriott. Not sure how the rooms compare.

The Langham is central and an easy walk over the river to the CBD, but still south of the river wouldn't be my choice. But look at the sights you are interested in, maybe it would work for you.

Anywhere in the CBD will give you access to free trams which makes getting around pretty easy. But the Southern Cross / Docklands area is not the ideal spot in the CBD for most attractions.