Half-Life
A pretty generic choice, but this has been my favourite game since 2001 when I first played it. While the game's engine isn't much to write home about by today's standards, I still find the game works just as well as it did back in '01. The game's well paced with great transitions between gameplay style, start with tense survival-horror at the beginning, to more tactical based gameplay when the marines show up, then everything thrown out the window when you get to Xen. The action is intense and well rounded, I found all the weapons useful in some way, and I liked how the story rewarded you for paying attention, but otherwise wasn't hard to follow.