The Google Cloud team revealed in their recent blog post that there has been a large scale DDOS attack on their system in September 2017, which was around 2.54 Tbps of traffic load generated by attackers. This number ranks the attack at the top of the list, dwarfing many other known and measured attacks. This first-place ranking is depending on the bandwidth of attacks as there have been measurements regarding packets per second and requests per second.
Target was not revealed
Another blog post/report on the same day (16th of October, 2020) from Google Threat Analysis Group claimed that the source of the attack back then has been from China, originated from some state-sponsored groups, and came from four Chinese internet service provider IP pools.
On their two separate blog posts, Google details the attacks, emphasizing on their success in mitigation and ways of detecting and backtracking the threats after they occur. Google claims that distributed denial of service (DDOS) attacks is growing exponentially and backs these claims with hard data. Google also discusses how different types of attack payloads (packets, requests, and bits) affect the services in general.
As many details have been shared on blog posts, Google did not reveal which system or systems have been targeted by that 2.54 Tbps DDOS attack. This is an understandable attitude, even when Google does not rely on security by obfuscation it is better not to reveal anything to the public on this matter and provide any data for possible social engineering.