GDAX is one of the better exchanges in the industry. It's bought by Coinbase a good while ago, and has them standing behind it. I must however point out that they have a very strict verification process that they are very tight with all the way through. If you have no problems with that, then definitely sign up there. You could also choose for Coinbase itself, but they in comparison with GDAX charge far higher fees, which might be a negative aspect if you're a high volume trader. At second place there is also Bitstamp, where you again will face a strict verification process, but that nowadays with all the regulations is something more exchanges are subject to.