Half-Life 2: 20th Anniversary Update

I never played it. What makes it being rated so good?

It's a bit dated now, but at the time it was revolutionary. A bit like the genre-defining movies such as Star Wars, the Matrix, or Terminator 2.

It's hard to put a finger on it, but it was a combination of a truly imaginative setting on-par with a decent science fiction movie or novel and the "show, don't tell" style of proper world-building. It presented a uniquely interesting dystopia, discoverable through a game instead of a movie or book.

I think it was one of the first games I played that was dark. For example, the "police" are cybernetically enhanced, but in a disturbing way ("transhuman"). The low-ranked troopers "just" have permanently worn facemasks a bit like Darth Vader, with an implication that they may not be functional without them. The higher-ranked shock troops have increasing levels of augmentations until some have only a single central "eye" in their visor. You shudder to think what's hidden underneath.

It was revolutionary when it came out due to its physics engine, character facial animations, graphics, and most of all it had a captivating story and great gameplay.