You could arguably call the 2008 meltdown the largest cyberheist. I can't find that article again but gosh was it great, an interview with a programmer who said about CDS something like we fed offal into a grinder and got steak out.
Ok, we'll s/World’s Biggest/Bangladesh/ above if people will refrain from complaining that other Bangladesh cyber heists are thereby excluded.
I find it hard to believe that the biggest plaque in the high roller room was a miserly $20k.