Possibly the best tactical turn based game on the PC.
The setting of this game is what makes it so original. You command an anti alien tactical team, you must research and build bases to counter the alien threat, slowly but surely the alien threat will accumulate and it is up to you to bring them down for the count.
There are a ton of things you need to keep track of. For example you need to have good relations with nations which fund your organization; you will also need to research weapons and items which will aid you in combat. The turn based combat is really a fundamental part of this game, and it makes you feel sucked in the action, because each turn can be your demise or victory. You feel vulnerable at all time, because you cannot foresee the alien's move, so it is imperative that you plan each move before executing it.
I play the game using the "X-COM Extender" mod with mp3 music files that enhance the old classic, and make it even more enjoyable. Many have tried to copy this formula, but even the best spiritual successors cannot beat the definitive original. A must have for any tactical gamer.