Here is my argument for Frankenstein: much of the great science fiction to follow, the entire "cautionary" branch of sci-fi, not only has Frankenstein in its genes, but remains directly derivative of it. William Gibson's entire Sprawl trilogy is essentially an updated Frankenstein. The Terminator franchise. Do Androids Dream of Electric Sheep/Bladerunner. Murderbot, one of my favourite characters in contemporary sci-fi, is essentially a riff on the Monster. And on, and on. I don't think any other work in sci-fi or fantasy has had as much cultural impact. Not even close.
And Mary Shelley was eighteen when she invented it. Mind-blowing.