Yeah the original is best, it is a strategy and management game with tactical (turn based) battles.
You build a base, equip troops, shoot down UFOs, send the troops to the crash site and kill the aliens/recover tech. You then research the tech and create new stuff based on the alien tech... when you get more money (from funding) you can build more bases around the globe. Occasionally, and more frequently as the game goes on aliens will attack cities, and if they find your bases they will attack them and you will have to defend. There is a plot which you uncover though research. XCOM is a global task force and you are funded by all the countries in the world, the amount of funding you get from each depends on how well each country thinks you are defending them from UFOs.
Its a fantastic game and you really get attached to your troops, you can decide what their names are (or go with defaults) their stats will improve with experiance and training and there is a ranking structure based on how many men you have. You research and create new body armour and new weapons, new devices, tanks, jet fighters and even space ships.