While the cover of the paperback that I read states that Bradbury is "The World's Greatest Living Science Fiction Writer", I respectfully disagree. Science fiction is so broad a field that there is significant overlap with horror and fantasy. I would without question call Bradbury the best author ever in the field of horrific science fiction. For, while his stories are generally based on a scientific theme, the real power is in the horrific aspects of the events.
When I was young, my favorite short story was "The Veldt", the first one in this collection of Bradbury's best short stories. A modern house contains what we would now call a holodeck, and instead of the children conjuring up delightful images, they are interested only in a scene of the African veldt, where lions pursue and devour their prey. Complete with the smell, sound and heat of the plains, the parents of the children are concerned that it is unhealthy. The parents try to do something to stop it, but they end up being consumed by the lions conjured up by the room.
Most of the other stories deal with the same theme, technology gone wrong. Atomic and biological warfare appears in many of the stories. However, the best part of all the stories is the tension and the unusual endings, often based on the frailties of human psychology. The intertwining of science fiction and horror makes these stories unique and I see a lot of similarities between Bradbury and Stephen King. In this area, he is better than King.