What's the iconic moment from the Half-Life universe? Perhaps it's being woken by the G-man's pep-talk in Half-Life 2 as your past and future flashes before your eyes.
After twenty years in limbo, you're on a train to alien-occupied City 17. Exiting into the monolithic station hall, a floating camera blinds you with its flash while Dr Breen leers out from the screens way above. The masked Combine soldiers bully you at the checkpoint; their batons descend when you try to flee. You're a stranger in a strange land, and only yesterday you were in the test chamber.
So perhaps it was the original Half-Life's monorail ride to the chamber and opening a door to another world. That slow build-up, that ominous hum, that crack as you were catapulted across the universe and returned back to a lab overrun by alien creatures. Or perhaps it was an altogether different moment - that one in Portal 2 where you were catapulted towards a test chamber wall by a co-op buddy who put that portal one foot out of place. What is that IDIOT Atlas doing? You're going to be shattered by the impact and rebuilt at the entrance again but you can't move your portals because Atlas will need them to follow you and the acid fizzes hungrily below you and the portal still isn't there. What the hell is wrong with him?
No, it's that moment with the strider on the rooftop, or getting the crossbow just before you meet the ichthyosaur, or crossing the bridge, or the hunters assaulting the White Forest Inn at the end of Episode 2. It's finding the first of the Ratman's graffiti-riddled dens, or Alyx's pet tank Dog tearing chunks off the strider in the woods, or fighting for your life with buzz saws and cinder blocks in Ravenholm. It's that bit where GLaDOS sung you a song, where you learned the cake was a lie, where you raced an airboat out of City 17, where you fought side-by-side with a girl you've grown to love.
And breathe. I don't have a singlestand-out memory of Half- Life; I have hundreds of them and they're all different, and they were probably different the way you played them, too. The series doesn't have one thing that makes it superb; it has many, has pioneered many, and will continue to have them as long as Valve has lines in its employee handbook like: "A fearless adventure is knowing what to do when no one's there telling you what to do" - and a team full of people who believe in that.
Valve doesn't know how to stand still. Their first game defined storytelling in first-person shooters and gave you the best action experience since Quake. The second, in 2004, gave you characters you could love, revolutionised environment design, and introduced the world to game-changing physics. Episodes 1 had the best AI partner ever, Episode 2 had the boss fight to end all boss fights. Portal and Portal 2 showed the world you could drop the guns and enjoy yourself just with comedy and physics puzzles.