It's hard to think of a game in the last 14 years which has manage to accomplish the same heady amalgam of fun, immersion, aesthetics, and variety. There's barely a limp moment anywhere in Half-Life 2 - but it's also startling how few games have been influenced by it. No wonder Valve has struggled to give birth to a bona-fide follow-up.
For now though - and possibly forever - Half-Life 2 remains the best video game I've ever launched into my crevice. Here are some of the reasons why.
The train journey into City 17 is one unbroken establishing shot, setting up where you are, the state of where you are, and saying so much without once resorting to a big exposition dump. It trusts the player to be clued-up enough to fill in the gaps, never resorting to heavy-handed, wannabe cinematic, storytelling techniques.
It also hints at an underlying mystery, as you're visited by the ghostly apparition of a besuited figure who became known as the G-Man. Friend or foe? Half-Life 2 leaves that up to you.
Though let's be honest... you should never trust someone in a business suit.
You meet your first friendly face in the form of Barney - an undercover member of the Resistance - who saves your life once everything starts kicking off. None of this sort of thing was standard when Half-Life 2 was released, but it remains a uniquely powerful - and unusually atmospheric - experience.
Before you get a weapon, we also get the first use of the game's physics engine - Gordon being required to build a pile of boxes to escape. Just 15 minutes in, and Half-Life 2 had set out its stall - this was a game unlike any other.
Barney's appearance is also the moment you realise that Half-Life 2 is next-level in terms of its characterisation. There's a weird Saturday morning cartoon quality to the acting in much of the game - Alyx and her dad aside, perhaps - but somehow it works opposite your silent protagonist. Gordon remaining mute is not only an opportunity for a few meta gags, but a way of allowing the player to embody him.
Why put words into your character's mouth when the player is that character? Again, Valve knew it was making a video game, not a movie.
From the towering Citadel, to the video screens, it's clear that this is a place that has been ground into subservience by a jackbooted, fascist, power. However, even before then, you get the sense that City 17 had seen better days. The juxtaposition of old world architecture and sci-fi trappings is never less than utterly believable.
The escape from the city - through apartments and across rooftops (still weaponless) - offers some of the best environmental storytelling ever seen in a game, while being a perfect exercise in escalating tension.
It's also an opportunity to show off the then-peerless game engine and unique production design. Think about it: Half-Life 2 doesn't look like anything else. The Combine troops, the dropships, the Striders... even the wall cameras - it has a unique feel, all tied together with a muted, earthy, colour palette.
Consequently, you stay engaged with the story.
It demonstrates that Valve never had ideas above their station - they weren't trying to compete with Hollywood. Like Nintendo before them, Valve wanted to make a great video game and nothing more. And the way to do that is to let the player remain in control.
When Dog seemingly sacrifices himself, attacking a Combine drop ship later in the game, it's heartbreaking.
Later, the device gets supercharged, and becomes capable of capturing enemies in its field - and flinging them like rag dolls.
The Gravity Gun alone would've marked out Half-Life 2 as a game of unprecedented imagination, but that same level of thought can be seen in every aspect of its design.
The sections in which you travel by airboat and buggy, along desolate roads and abandoned canals, are some of the best moments in the game. Not only does the driving feel right, but the fact you can stop your vehicle to get out and explore buildings, helps that sense of immersion.
One of my favourite things to do in Half-Life 2 is explore the old shacks along Highway 17. I must've spent hours using the gravity gun outside that one house with the tyre swing.
It's also the area with the aforementioned saw blades and paint tins, put to entertaining effect by slicing enemies in half, or covering them with paint. It highlights just how fun Half-Life 2 is to play - Ravenholm is a sandbox for the Gravity Gun, empowering players with the freedom to just go nuts.
Instead, you have to think about how to defeat them - keeping them at bay by activating Restrictors, which pound the ground, creating a sound that the Antlions hate. Making your way across the beach requires a degree of planning - step foot on the sand and the Antlions will emerge. Fortunately, the Gravity Gun can be put to work constructing makeshift bridges.
Coming midway through the game, the beach level demonstrates how Valve sought to keep things fresh for the player, introducing new gameplay elements and environments for the entire duration of the game. Half-Life 2's greatest strength was its ability to remain surprising.
It takes place a week after the preceding stage - set in the bleak Nova Prospeckt - following a botched teleport. There has been an uprising, and you find yourself in the midst of all-out war between the Resistance and the Combine.
It is, appropriately, a consolidation of all that has come before; though significantly more action-heavy than a lot of the game, it still weaves in puzzles, tension, and storytelling. There's an astonishing sense of you being just part of the story here - that all around you, the NPCs are engaging in their own adventures.
The epic conclusion takes place in the Combine Citadel which has loomed, both metaphorically and actually, over the entire game. Still keeping the gameplay unpredictable, Half-Life 2 suddenly becomes as much platformer as it is shoot 'em up - albeit a shoot 'em up with a now-supercharged gravity gun, which allows you to rip out the fittings to be used as ammunition.
Admittedly, the final challenge - destroying the Citadel's reactor by firing energy balls at it - feels slightly mundane compared to all that came before, but when all that came before is virtually peerless, almost anything would've been a comedown.
The game concludes in an open-ended way, with a return appearance by the enigmatic G-Man hinting at future adventures - and the suggestion that everything you have done has been at his bidding...
Half-Life 2: I want to lay my eggs in you.