That OPM hack was pretty brutal. It set back clearance investigations for years. They still take twice as long as they used to. All of the contracted agencies got fired and everything started over. Effectively every detail of my past, personal life, medical status, and everyone else with a clearance has been in the hands of China for 6 years. The SF-86 is extremely thorough. It centralizes much more of your personal data than any other source. A lot of this is stuff you would not normally disclose anywhere.
Other than that, I've give a vote to the original hack, the Bletchley Park cracking of Enigma. Most of the other hacks have just hurt people and exploited systems with poor security or no security at all. These people helped bring down Naziism and broke what was the best military grade encryption in existence at the time.