Well, personally I think The Conversation is THE great American film, perfectly directed and Hackman's performance is stunning. He's never bettered it and when you look at his body of work, it just shows what a brilliant actor he is.
The Conversation is obviously based on Antonioni's Blow Up, but it improves on that film in every way, and this is from a huge fan of Antonioni. I think it's perfect, it has one of the most haunting endings ever put on film, and a lot of that is down to Hackman.