X-COM: First Alien Invasion
This is a scaled-down, greatly simplified version of the original.
Because of the lackluster space combat, Interceptor is the first X-COM game where the micromanagement and research portion is the more satisfying of the two halves.
X-Wing Collector's Series
These are classic games that belong on every computer gamer's shelf, no doubt about it.
X-Wing vs. TIE Fighter: Balance of Power
Balance of Power does an admirable job of addressing almost all the complaints gamers had with the original game.
This is almost the X-COM sequel that folks have been hoping for.
X-Men vs. Street Fighter
I like the Street Fighter series as much as the next guy, but this is taking it way too far.
X-COM: Terror From the Deep
Apart from new art and a handful of new combat options, this is exactly the same game as UFO Defense, only much more difficult.
X-COM: UFO Defense
Put simply, X-COM is a bona fide modern classic, standing proudly alongside Civilization and Populous as a benchmark in the evolution of strategy gaming.
X-Men: Children of the Atom
Proves that the PC is indeed a viable platform for fighting games.