Tic Tac Toe

Three in a row, done properly. The computer plays a full minimax search of the game tree with no depth limit, so on Impossible it genuinely cannot be beaten - noughts and crosses is a solved game, and the best anyone can do against perfect play is a draw.

It picks at random between moves that are equally optimal, so it stays unbeatable without becoming predictable. Easy and Medium are the same engine playing its best move only some of the time, which is what a beatable opponent actually looks like - one that makes mistakes, rather than one following a worse rulebook consistently enough for you to read.

There is a two-player mode for the same screen, a running scoreboard, and the opening move alternates every round so nobody keeps the first-move advantage. Marks draw themselves in, the winning three are struck through, and everything is timed against a clock so it looks the same on any monitor.

Tap a square, or use the number keys.