Sokoban

Sokoban is one of the purest and most influential puzzle games ever made. Originating in Japan in the early 1980s, its name translates roughly as "warehouse keeper," and that's exactly what you are: a little worker in a cramped storeroom who must push crates onto their marked target squares. The rules could not be simpler. You can only push, never pull, you can only move one box at a time, and you can shove a box only if there's empty space behind it. From those few constraints comes a staggering depth of difficulty.

The genius is how quickly "simple" turns into "fiendish." Because you can't pull, a single careless push, jamming a crate into a corner or against a wall where it can never be freed, can render a whole level unsolvable, forcing a restart. Later puzzles become intricate spatial logic problems where you must plan a long sequence of moves and think several boxes ahead. There's no timer and no action, just you, a grid, and the quiet satisfaction of untangling a knot that looked impossible.

Its influence is impossible to overstate; the box-pushing mechanic appears as a sub-puzzle in countless games to this day, and Sokoban itself has been reimplemented on virtually every platform ever built, including the MS-DOS version here. It hasn't aged a day, because there was never anything to age. Minimal presentation, infinite challenge; it remains a benchmark of elegant puzzle design.