Northern Beaches: Narrabeen, Little Narra, Dee Why Point & Curl Curl.
Eastern Beaches: Maroubra, Tamarama when it's big & working.
Southern Beaches: Cronulla Point, Voodoo.
Respect the locals, particularly those areas with heavy local scene: Maroubra, Narrabeen etc.
Just out of the city, visit Garie Beach in the National Park, seldom crowded, awesome scenery. Travellers can also stay at the YHA there: http://bit.ly/H4N49 almost 7 years ago