There's a beautiful night time view of Sydney that is sort of a local's secret: Blues Point Reserve at McMahon's Point. You not only have a fantastic view of the harbour, the bridge and the opera house, but you also get Luna Park as well (which just looks amazing lit up at night). I take every guest I know who comes to Sydney there for a look. It's superb.
I'm fond of the view from the Opera House - but I also like the view when you are walking to the train station from the Domain (after the summer concerts) - it's a little different, but pretty nonetheless.