Hi,
I'd have to say The Orange Box, which isn't really one game but five in one: Half-Life 2, Episode 1 and 2, Portal, and Team Fortress.
The Half-Life series (and especially the story arc of number 2 and its Episodes) are quite possibly one of the most revolutionary games ever to touch the PC. With amazing interactivity, ground breaking graphics, and an epic storyline, they take the cake in terms of, well, almost everything.
Team Fortress is fun, but what really makes Orange Box stand out is Portal. A short, rather simple game, it bends the mind as you jump through portals and solve complex puzzles. It sounds sort of stupid, but you really have to play it to understand its addicting nature. It's quite funny as well.