There are lots of good exchanges and some of it are being mentioned already in some of the responses.

Just want to asked some question so that we may direct you to the exchanges that will fit your needs. Are you going to used exchanges for trading purposes only? Or you also want to do withdrawals (converting it to fiat) there? Are you a type of guy that's fine in terms of providing identity (for limit purposes)? What country you from and how do you plan (payment method) to put money on the exchanges?