CoinBase is the safest, as far as Im concerned. Bittrex, Kraken, Blocktrades, et/al, do not have the federal SEC oversight that coinbase does. I had to sell some litecoin to pay for insulin and kidney medications, and I had the funds sent directly to my bank account via Paypal from coinbase in less than 30 minutes. It was smooth compared to Kraken and Bittrex. Poloniex is no longer available to US customers, also, even though it is based in Delaware.