There is no site that is 100 % secure. If you just want to store Bitcoin Xapo is probably the most secure. If you want a trading platform Kraken is probably the most secure. However, the interface of Kraken is not very nice. You can also read about hardware wallets, I have no experience of them.

I store my Bitcoin at different online wallets to spread the risk. I have experienced Mt Gox, Bitfinex and Bter hack/shotdowns. If you want to read more about different online Bitcoin exchanges you can visit www.cryptocoinzone.com