They revealed the existence of PRISM and afterwards published lots of analysis on the Snowden leaks. I believe they were also involved in the Panama Papers and I believe they broke the news about a UK/US black site and some war crimes after the invasion of Iraq
And Spy Cops: https://www.theguardian.com/uk/2011/jan/09/undercover-office...
The BAe bribery scandal: https://www.theguardian.com/world/2003/sep/11/bae.freedomofi...
And before that the downfall of Jonathan Aitken and Neil Hamilton in the 1990s.