We probably can never know the biggest losses, as they probably occurred in private/semi-private games such as those in Macau and Monaco. However, we do know that Andy Beal has some fairly well documented multi-million dollar loses. The most famous of which was the subject of "The Banker, The Professor, and The Suicide King." Then there was a staggering loss to Phil Ivey and the Corporation of some $16.6mm in 2006. The most recent a reported $5mm loss to Todd Brunson this year.
Cheers!