X's & O's. The worst game I ever owned, and yet, the only one I miss after selling it. I've sold much better games that I simply had played out. But X's & O's was something special. It is a sharpshooters game. Short tight shots, and missing by a little bit would cost you the board. No flailing allowed. So the game made me play better than I did.
(Note: It would have been LOTR, but I finally bought another one)