Zork would have to be one of the all time greatest text adventure games. One of the greatest things about the Zork series is that it is based on the Zork books – which themselves are ‘interactive fiction’ – in that you get to make decisions as you read that affect the outcome of the story. Reading the books can actually provide valuable insight as to how to successfully complete the game. (UPDATE: Apparently I got the chronology wrong here – and the books are actually based off of the game. Either way they make a great companion set)
One of the best known quotes from Zork is ‘It is pitch black. You are likely to be eaten by a grue. These days it seems kind of tame – but I can still remember being terrified when this text was on the screen
You can download a copy of Zork and relive the experience from here.