Going off memory alone, back in say 2004-2005..? There was a guy who was considered very trustworthy uploaded a client to the rs-tools section, which contained a keylogger.
He then proceeded to loot every account over a period of a night and was said to of made 10-20B maybe even more (bare in mind the cost of gold in them days).. He then dumped all the account information for every account he had on the forums which then led to everybody getting their accounts stolen before they'd even realized they'd been scammed. I'd say for most people affected and collateral cost this would be the one that takes the prize..
I was personally involved in this and hacked myself, and to those that will accuse me of ban evading I had another account from 2003-2004 time before everyone's accounts were reset