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

They have a search tag for it, although it's also used for articles about investigative journalism so you have to scroll a few pages.

If you just want examples, the articles about various journalism awards often list topics from that year.

https://www.theguardian.com/media/investigative-journalism

Thanks for these replies, I may "buy" a subscription!