Enthusiasts agree that science fiction's Golden Age began with A.E. van Vogt's short story "Black Destroyer," published in July 1939, two months after my father was born. You can read the story online, and you'll recognize a lot in it.
So many SF stories have followed in its tradition, but few have done the most interesting thing van Vogt tried: he got inside the head of the lone alien monster, and told most of the tale from its point of view. The atomic power in the story seems a bit hackneyed now—but recall, it was 1939, six years before the Manhattan Project.
Van Vogt, a Canadian, lived a long time, and he saw real voyages through the solar system. He died on January 26, 2000, coincidentally the same day my younger daughter was born. And "Black Destroyer" is still one of the coolest SF short story titles I can think of.