"Yes. It's been done before. In fact, it's been done to death. Let's dare to pick the project up one last time... And finish it."

 
        x | o | o
       ---+---+---
        - | x | -
       ---+---+---
        - | - | -
       I took 0.000001 seconds!
       Make a move (1-9)
       9 8 7
       6 5 4
       3 2 1
       >>_
 

A perfect-play tic tac toe engine plays a game with no errors. The game will never yield a win for the user. A perfect-play engine is based on the classical Minimax algorithm. This engine is a perfect-play engine.

The game of tic tac toe is played on a board with nine squares. This conveniently allows us to represent the board with two 16-bit integers (four bytes). Marking and erasing a move on the board is accomplished with xor. Tie checking is accomplished with a simple equality check. Victory checking is a matter of mapping one layer of the board to a single bit in a magic table.

Due to its extremely small state-space, the full game tree of tic tac toe can be explored in a matter of microseconds. This speed allows for exploration of the entire "perfect-play" tree in well under a second. The perfect-play tree can be used to create a table of perfect moves. These perfect moves are precalculated at startup and stored in a 12288 byte table with an "open-address" hash scheme.

I think that the time and space complexity of the engine can be improved with minimal perfect hashing. However, I was unable to find an easy method (not enough time on my hands, unfortunately). If you can find a method, please let me know and I'll add you to the project!

1 microsecond.