There are many great games and for many people it is hard to choose which game deserves the very first spot. But for me it is no difficult choice at all, Half-Life (1998) easily gets the first place spot with no close competitor.
Anyone who does not recognize Half-Life as the top game, or at least as one of the top games ever made I consider such a person to be a video game fool who has no understanding of gaming history and the influence of which games brought.
What makes Half-Life so special is how it revolutionized the first-person genre completely. FPS games fall into pre-Half Life and post-Half Life era. Consider FPS classic games like Wolfenstein, DooM, Quake and Duke Nukem. All of these games are classics and a game like DooM or Quake would probably even make the top 10 list. But all of these classic shooters lacked something.
All these shooters were pure action games. There was hardly any story. Sure the game had a story. In Wolfenstein you were escaping from a Nazi castle. In DooM you were fighting an invasion from Hell. But that was it. None of the story developed. Each level was the same. It was just more intense. You collect keys, or cards, to open doors and move on. That is not really a modern FPS game, that is an old fashioned FPS game that is purely action/arcade based.
It took one game to change them all, and that game was Half-Life. Half-Life had a revolutionary different way in which FPS games were presented. For one thing in Half-Life the game started before the story. You began playing as if nothing happened. All other games put you into the middle of the action. Half-Life allowed you to see how the entire story develops from even before the beginning.
There were no levels there were no cutscenes there were no messages boxes to tell you a story. You get to see the story. You get to experience it. You develop the story with your actions rather than the story being handed over to you.
The story itself was superb. The ending was special, in that you choose your own ending. The gameplay was fantastic. The artificial intelligence was amazing. The graphics were decent(for its time, though this was the weakest part of Half-Life, however, lucky for me, I always say that graphics do not define a good game). All of these elements together created the finest game that were made.
Just consider the following fact. When Half-Life came out it got over 50 "game of the year awards". PCGamer called it to be the greatest game of all time in its 2000 and 2004 edition (not sure about other editions, if they ever even had those). GameSpy had a 64 playoff battle between all great games which were decided by voting members. Without a surprise Half-Life won the first place followed by Super Mario Bros. Recently GameSpot (not 8 years ago, recent, 2010) had a playoff between the best video game heroes, Gordon Freeman (the charachter from Half-Life) got the first place. Consistently Half-Life dominated these sorts of contests.
There is a reason why this is the most decorated game in history, because it is the best. The problem is that games are not like books. A game which is three years old is already considered to be "old". Half-Life is 13 years old, I remember playing it as a little boy. So it is largely forgotten by a lot of people. It has become forgotten how revolutionary this game really was and how much it influenced all following first-person shooters. Whenever you play any modern first-person shooter there is a remnant of Half-Life in all of them.
Of course, it is possible to make a modern game today with a 10 gigabyte memory, powerful graphics, and a stronger AI that is better than Half-Life. That is not amazing. What is amazing is making a game like Half-Life back when computers were not so powerful. And it is not so amazing because it will be influenced by the structure of Half-Life itself.