You will get lots of suggestions for "the best place" that reflects the experience/travels of the posters.
I would suggest you come up with a list of the types of sharks you want to see. Certainly here in Sydney there are sites within newby range where you can almost guarantee certain species. Port Jackson sharks, Wobbygong and Dusky Whaler sharks are common. Grey Nurse sharks (called snaggletooth sharks in some international locations) have aggregation sites here and close by. Great White Sharks cage diving is commonly done on the West coast. There are successful Whale shark diving destinations. Other areas will offer various types of Reef sharks, lemon sharks and tiger sharks.