This was my ~6th playthrough but only now did it hit me in a way I went "Damn, this really is one of the best games ever" as the final cutscene and the end credits played.
On one hand, it is a decent puzzle game with the portal mechanic. No longer unique, especially after multiple playthroughs, but still so much fun.
And of course, what makes it even better and still great today is everything around the gameplay. The atmosphere and the whole dark comedy of the game.
At first, this playing in the Half Life universe seems like a neat little afterthought. It becomes unnerving once you learn it definetely plays after the Combine have crushed most of Humanity. GlaDOS mockingly asking you what you even want to do outside. Saying "I'm the only thing between us and them. Well, I was." It uses that universe to create an existensial dread without having to establish it.
And I have not even adressed the comedy and horror from inside. Minor malfunctions in the AI slowly turning into actual threats and antagonizing. The test chambers slowly showing cracks and giving a window into an industrial horror. A death trap with no humans except you left, and only a mad machine left to run the place.