Just wanted to remind folks here that Half-Life is the game that single handedly popularized the entire genre of First Person Shooter (FPS). That in itself is a big achievement but the game is much more than that with its intricate puzzle style with giving minimum instructions to the player and let the player figure out things.

Edit: OK...I see folks reminding me about other classic FPS games that came before HL but I was too young when those games came out so my first FPS experience was with HL. Also by the time I started playing video games those games were too old graphics-wise to entice me to play. But fair point!