from some of the replies i have seen so far, i have to wonder how much experience some people have with games
in my mind, Half-Life is the best computer game made to date. few, if any, FPS games can match the story (heck most games of any genre don't have a story comparable to HL), the gfx were great for the time, the multiplay still holds up today-nearly 6 years after release. it launched the modding craze. it reinvented how the FPS single player game was setup. it continues to be the FPS game that all games are compared to. graphics aside it is still a GREAT game and could be released today successfully.
of course FPS games do not make up the entire game market. however i am hard pressed to think of a better overall game i have played or heard of on the computer. (my knowledge of console games is severely lacking). some other notable PC games in my mind:
Civilization 2: great strategy game that may have been surpassed by successors (alpha centauri, Civ 3) however i lack first hand experience with the newer ones. the unlimited replayability of this game combined with the balanced gameplay is only hindered by the lack of multiplay support. of course who wants to play against others when it means waiting possibly 10 for each turn, up to 8 players. being it was released before the multiplayer exploded, this is forgivable.
Warcraft 2: my personal favorite classic RTS. warcraft and C&C basically brought RTS to the mainstream and did it in style. the two were cloned by wanabe's for years until (Blizzard's own) Starcraft came on the scene. even then the genre stayed mostly the same until the recently released Rise of Nations.
Doom: not exactly the adam and eve of FPS games, but you can credit it with the successful birth of the FPS in the mainstream and giving birth to successful multiplay.
diablo series: sure they may not be the greatest RPG's in the classic sense of the phrase, but millions of people can't be wrong when it comes to the pure fun factor of these RPG-lites. even today noone has been able to successfully clone the diablo style of gameplay, and with battle.net support (addmittedly at times being weak) multiplay gave the games nearly infinite life. without all the complex numbers games of more hardcore RPG's these games are just plain fun.