Some examples off the top of my head:
1. The JFK files classified for 70+ years.
2. The justification for the Iraq War.
3. Post 9/11 electronic surveillance of Americans
4. The Epstein files.
Now, the libertarian in me would like full transparency. But the pragmatist in me realizes that some information must be protected/obfuscated/manipulated for national security reasons. The partial list above are cases where we deserve full transparency and accountability, IMO.