Half-Life, the original. It's focused, intense, and has just the right amount of environmental storytelling to let your imagination fill in the rest without taking away from the running and gunning experience. HL2 is fine, but I never cared for the human resistance aspect of the game. To me Half-Life is at its best when it's about the one man's war against all the odds. I liked the new puzzle aspect of HL2 and all the interesting ways the player is let to use environment against the enemy, but I did not like driving/boating sections with constant obstructions that felt like busywork.
