It also depends on what you're looking for. Most shopping centres (Westfield, Macquarie Centre) have the usual run of major stores, but there are also specialist centres such as DFO for more outlet style stores and Homemaker Centres if you're looking for larger items (but I presume not).

Macquarie Centre is a good size with a wide range of stores and eating places. It's also easily accessible from the CBD by train/metro and bus (slower and less comfortable).

If you want to make a day of it, Macquarie Centre is a short bus ride from North Ryde shopping centre and a bit further still to Rhodes which has a good Vietnamese style food outlet doing decadent Banh Mis with fresh roasted pork, rice noodle salads and rice paper rolls.