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.