Ikaruga

Ikaruga is the shoot-em-up that Treasure built around a single mechanic: your ship can flip between black and white polarity at any moment, and bullets of your own color pass harmlessly into your hull, charging the homing laser. Bullets of the opposite color kill you in one frame.

The scoring system is even more demanding. Destroying three enemies of the same color in a row triggers a chain, and every level is laid out so that maintaining that chain requires switching colors in a specific rhythm — solve the puzzle and the score multiplies, miss a beat and the chain breaks until the next set of three. Five stages culminate in boss fights that demand both pattern recognition and improvisation.

Three difficulty levels, two-player local co-op, and a Double Play mode that lets one player drive both ships with a single controller round out the package. Vertical screen mode is supported for rotated monitors. Replays can be downloaded directly from the leaderboards, which is how Ikaruga still teaches itself to new players two decades later.