So here we go:

Diablo  Think about it: What do you do in Diablo?  You click on little monsters again, and again, and again, and go through tens of hours of busting barrels looking for better shit.  Why is that fun?  I dunno, but it is.    Diablo epitomizes the hack-n-slash RPG sub-genre, and spawned numerous clones and highly successful sequel.

Fallout  Fallout is heavily story-centered in the finest tradition of Occidental role-playing games.  In a daring move, Fallout shed the high fantasy setting typical of CRPGs and dropped the player in Mad Max-inspired, post-apocalyptic California.  While on a quest to save your home village, you as the player encounter an array of towns and settlements, with their own problems and conflicts.    This was particularly stunning in the light that some of the decisions the player can make are morally ambiguous.  There have been few RPGs since that gave the player such a great responsibility to the story.

Master of Orion II: This is the ultimate 4X game.        Popularized by Civilization, these games put the player in charge of a tribe, empire, planet, or some such, giving him complete control over every element of that society.  Technological research usually plays a key role in these games, as does colonization, infrastructure improvement, and military conquest.    In Master of Orion II, called MOO2 by fans, the player takes control of a single planet within the galaxy, competing against several other races with similar meager beginnings.      Better yet, when two enemy fleets meet, the game goes to a tactical mode, where you get to command your recently-designed ships.    The tactical mode is not terribly deep, but it gets the job done.  The inclusion of both a grand strategy and a tactical mode in MOO2 predates Rome: Total War by some eight years.

I should probably put Counter-strike on this list, except that as much as I hate to admit it, I never seriously played it.  So, sorry; no Counter-strike.