The rules
X moves first. Take turns claiming an empty square; the first to get three of their own in a line — across, down, or diagonally — wins. If the board fills with no line, it is a draw.
Nobody can win at tic tac toe, and that is the point
Classic 3×3 is solved. With correct play by both sides the game is always drawn, and every one of the nine opening moves draws — the centre is not secretly winning, it is merely the hardest to answer. There are exactly 255,168 possible games: X wins 131,184, O wins 77,904, and 46,080 end level. Our move generation is checked against those published numbers, along with the 5,478 legal positions the board can reach.
So the honest thing to do is not to sell you a "hard" tic tac toe bot. On Hard, ours plays perfectly and cannot be beaten — only held. What we can offer instead is the versions of the game that are not exhausted after one sitting.
Misère — the same board, a different game
Three in a row loses. It sounds like the original in reverse, and it is not: our solver finds that only the centre opening holds the draw, and all eight other first moves lose outright against perfect play. Most people have never played it, and it undoes every habit the normal game teaches.
Ultimate — nine boards, never solved
Nine small boards in a 3×3 grid. The cell you play inside a small board decides which board your opponent must play in next: play the top-right cell and they are sent to the top-right board. Win a small board to claim its square on the big board, and claim three in a row to win. If you are sent to a board that has already been decided, you may play anywhere — which is the move that decides most games, because handing your opponent a free choice is usually a disaster.
Ultimate has never been solved, so we do not claim our bot is perfect at it. It searches, and the interface says so. The board shades whichever small board you have been sent to, because losing track of that is the single commonest way to blunder.
Free review
For classic and misère the review is exact: every move you played is compared against the solved value of every move you could have played, so "Best" means provably best, not best-looking. For 4×4 and Ultimate we tell you the position is too large to solve rather than grading you against a guess.