1998 Game of the Year, Half-Life was developed by Valve Software. From it's release, Half-Life went on to win over 50 "Game of the Year" awards from various publications. This of course was no suprise, Half-Life introduced a whole different world to the FPS gamers, and is still a benchmark for even the most recently released games.
Furthermore, Half-Life's success didn't just fade out over time, the more creative fans in the community took it upon themselves to keep the game alive for many years by developing their own modifications, some of which were almost more of a success than the original title - can anyone say Counter-Strike?
In conclusion, Half-Life set new standards, and then proceeded to break them itself, what more can you ask for in a game?