I'd go with DOOM -- it started it all, and is underrated by critical snobs -- though GoldenEye 007 and Deus Ex both get nods from me (the first for helping to sophisticate the genre, and the second for its marriage RPG and FPS elements and unsurpassed in-depth gameplay).
. . .As for Half-Life? A great title, for sure -- its seemless integration of storytelling helps maintain the game's immersion, and alot could be said of its multi-player add-ons -- but I always felt that its survivalist gameplay was a tad retrograde when compared to, say, GoldenEye, which had actively integrated objectives into its mission structures a year earlier. To this end, I think that alot of HL's appeal has more to do with presentation -- compare any level in it to Deus Ex's hokey inner-city environments -- but that it's not necessarily the paragon of FPS design.
. . .As for Half-Life? A great title, for sure -- its seemless integration of storytelling helps maintain the game's immersion, and alot could be said of its multi-player add-ons -- but I always felt that its survivalist gameplay was a tad retrograde when compared to, say, GoldenEye, which had actively integrated objectives into its mission structures a year earlier. To this end, I think that alot of HL's appeal has more to do with presentation -- compare any level in it to Deus Ex's hokey inner-city environments -- but that it's not necessarily the paragon of FPS design.