No other game can claim what Doom can claim though. That is to say, back in the day you didn't call them 'first person shooters,' instead you always heard them called 'Doom clones' and I think that really says something about the importance of the game. I have to say though, that there is always a game to make generational leaps ahead of its predecessors. Doom was the start, Quake revolutionized the genre and branched it out, while Half-Life took an entirely different path by preferring nuanced story and puzzles interspersed with intense combat in an environment which at the time I could only describe as "massive." Half-Life 2 continued that path and capitalized on physics interaction. I think the next big change was with STALKER and its smooth meshing of FPS, RPG, and adventure all crammed together into a (rather buggy) package. If I were to use the very loose and literal meaning for FPS, I would consider Descent and Portal as being a part of the continuing FPS revolution. Descent was incredible in being a truly three-dimensional environment where you could move in any direction. A more 'traditional' approach to that would come in Prey which was a good way to blow chunks late at night due to the mind bending physics. Portal though, to me, was one of the best times I've had in a game. I guess I had gotten so used to (older) 2D puzzle games that with Portal it brought the Half-Life puzzles into an entirely new world. Finally, a game that revolutionized the genre again was Garry's Mod. It's the sheer ability to do absolutely anything that is the attraction there and that I think is an incredible feat, especially for what started out as a free mod. Though, just like the original, it still has its share of bugs which seem to fluctuate between every update in seemingly random ways.