Half-life (1 & 2). I say this not for the games themselves (which are impressive and would easily contend for such a title), but because of the lineage of player mods and games that have spawned from them and their players. The half life series has had the greatest effect on the modern PC gaming community, without a doubt.
Starcraft, and probably Starcraft 2 when it comes out, I'm ashamed that AT LEAST 75% of the people who play WOW doesn't know Blizzard made RTS's, and im shure quite a few don't know what an RTS is.
I play WoW and I know that Blizzard made Starcraft. And RTS stands for Real Time Strategy. On the topic best computer game is World Of Warcraft:Wrath Of The Lich King
Diablo 2 was a pretty nice game, I played it for a while and it wasn't actually half-bad. But since i was playing at a computer cafe my character data kept getting deleted and i always had to start all over again...
This is because it has a great story line, also about 20 free mods have come out with it, and a couple you have to pay for, Like garrysmod which is a great game.
hmmmmm this is a tough one, currently I'd have to go with the WoW series. However Warcraft3 has lasted me the longest simply because of the amount of new games that people still make. Tower defenses, RPGs, Survival, even FPSs. There will always be something entertaining to do on Warcraft3.