For years I thought that All The President's Men was the best 70s conspiracy thriller. However, since I bought this film on DVD I have changed my mind.
I first saw this film on TV years ago and I was quite impressed. The film was listed as film of the week in the weekend papers, so I taped it. Since then I had been wanting to give it a second viewing, but it wasn't available anywhere.
The film itself draws parallels with the various political assassinations of the 1960s, particularly the death of Robert Kennedy. From here you'll have to watch it yourself, but Pakula builds a thriller based around secretive organisations and paranoia.
Well worth viewing, and it gets better everytime. You might need to watch the ending again to understand it.