Cryptsy was estimated at 6 million
https://www.hackread.com/cryptsy-hacked-bitcoin-worth-usd-6-million-stolen/
and Gatecoin worth 2 million
http://www.coindesk.com/gatecoin-2-million-bitcoin-ether-security-breach/
but Bitcoin itself has never been hacked as far as I know
Yes, there were several thefts around 6M$ but based on my research Picostocks was the biggest one. To add some more there were tens of cases of thefts around 1M$ and more since 2013.