Half-Life is one of the most important games to be released ever. It features a flowing storyline, without missions or levels. The plot becomes very dark and the atmosphere is consistent throughout the entire game. The enemies are creative and simple, but mixed with the environment creates very complex situations.
The graphics are outdated by modern standards, but back then they were top notch. The sound quality is great, especially the sound of old pipes and creaking metal that warned that the ground the player is standing on might collapse any second.
The best part of the game is the plot. What starts as a science experiment ends up involving military helicopters and other things that would spoil the game (although most people probably have played the game already).
This game is a must have classic.