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.