Praser 5

By far the hardest interactive fiction I have ever played. It is just a series of puzzles, represented by characters. Puzzles include Euclidean geometry, wordplay (similar to cryptic crosswords), and a maze.

Some of the puzzles are bewilderingly difficult (such as the name of the Mark of Water). I don't think that it was ever intended to be solved without cheating. Using a decompiler gave a few answers that I could not otherwise get.

This is a pure mental-exercise game with no plot. I've brought up some of the puzzles on Stackexchange and Reddit, and it provoked some good discussion, so this is a good game to mine for interesting puzzles.

This game could be likened to exploring the Moebius strip while breathing in Haiku. Plotkin has an incredible mind and the only way to beat this game, is to unravel it. I am addicted to solving the Griffon and Sphinx's riddles. I have not succeeded yet... but, the game is so simple and so elusive that I can't put it out of my mind. It may be strange to see I've rated the game before having finished, but - I don't need to finish my dinner to know that it is and will be satisfying to the end. What I appreciate about Praser 5 is that it has reactivated my own mind. It got me thinking and problem solving. Praser 5 will turn your mind in on itself, will shift planes and turn back again. It's a great mental calisthenic.
Note to Plotkin: have pity for the simple mind - even the hints need hints.