It's hard to give an answer to what the "greatest" cyber attack was that everyone can agree on. Some attacks are measured in monetary amount while other attacks steal information which is harder to gauge with a dollar figure but can be measured in the number of victims affected.
T think so far the "greatest" attempted cyber attack that would have affected millions of people and stolen nearly $1 billion in U.S. dollars if it had been fully successful was the Bangladesh cyber bank heist on February 07, 2016. Below is a YouTube video link describing the event. As you'll see in the video below the cyber attack against the Bangladesh central bank had been planned for at least nine months and each step of the heist was calculated to benefit the hackers and to thwart any attempt to stop the heist.. Fortunately in the end the hackers only got away with $81 million U.S. dollars of the attempted $951 million U.S. dollars.