This 17 year old game is deeper than half the titles on the market, and deserves its crown as best PC Game.
The gameplay is deep, addictive and atmospheric, and does a surprising amount for a game from 1993. Your task as the newly formed project for the defense of earth from Aliens is to stop alien raids and abductions by shooting down their ships, ambushing their landing parties, and destroying their bases. Eventually you are tasked with taking the fight to the aliens and defeating them altogether. There are two modes of gameplay, the Geoscape and the Battlescape.
In the Geoscape (real time, variable time speeds), you build bases around the world, dispatch interceptors and dropships to deal with UFOs, do research, manufacture and purchase weapons and vehicles, and try to please all your sponsor nations so they don't reduce their funding. However, it quickly becomes apparent that both human technology and the funding from sponsor nations are both completely inadequate for the task. In order to survive and stop the aliens, you need to capture alien technology, research it, and produce your own copies, and then sell the excess loot for mounds of money.
The Battlescape on the other hand (turn based tactical, top down) allows you to control your squad of soldiers (who improve over time in surprisingly sophisticated ways) in attempts to stop the aliens. But don't think it will be easy. The Battlescape is an unforgiving world filled with aliens waiting to snipe at your men from distant cornfields and ambush you from around corners. The first several missions you play, it is likely that you will be fail spectacularly. This is entirely intentional, and really contributes to the atmosphere of the game. You really feel like you are learning on the job, facing a totally unknown and technologically superior enemy who holds all the cards. A genuine feeling of satisfaction accompanies finally being able to defeat the aliens in a mission (albeit with heavy losses), or re-engineering your first bit of alien weapon tech. The lack of a tutorial actually helps the game in this way, as it is much deeper than it first appears, and it feels very authentic to learn the ropes of the game in this way.
The graphics and sound are marvelously retro, and might take some getting used to for some of the less open-minded gamers out there, but I assure you the reward is worth it. Overall, this is a game that you MUST play if you call yourself a PC gamer at all. It joins a myriad of other gems from the 90's that modern gamers are continually ignorant of, though they inform modern developers every single day. There is no better indication of the current stagnation of game development than playing a game like XCOM and seeing how incredible it is. This game with an extreme facelift would by strategy GOTY.