The biggest scam would be one that has yet to be discovered! However, in 2015 a lottery scam was revealed and its perpetrator arrested. This was Eddie Tipton, who had amassed approaching $Au32 million in 5 wins. He worked at the Iowa based Lottery (MUSL) and bipped the coding so that his odds zoomed from 5 million to one, to 200 to one. 5 times this paid off, but he continued in his job using other people to buy his tickets.
This way it avoided identifying him, but on the last occasion he bought his ticket himself and although he used someone else to pick up the winnings almost a year later, the rule that winners must be identifed cinched his downfall. He'll serve around 20 years for that scam, or the full 26 if he doesn't behave well.