I could list off comparisons: Karfka, Lovecraft, especially Blackwood. The real word I want to use is dread. 
 Annihilation is a masterpiece of dread, of that feeling that the world is wrong. "Unsettled" works pretty well. Whether in the growing horror at the setting of Area X or the discovery of how many lies have been told to our protagonist, you can never safely settle in. The reader is adrift on the quickly moving current from start to finish. 
 The characters in Annihilation have no names, just job occupations. A biologist (our protagonist), an anthropologist, a surveyor, and a psychologist embark on a government mission into "Area X," a strange wilderness inside a mysterious boundary. From the first moment, we lack the normal things to latch onto. The team is unsure how they even entered the area, and this is emblematic of Annihilation, a book where our hero spends the book grappling with the unknowable. 
 This is Roadside Picnic if it was written by Algernon Blackwood. Lovecraft said that Blackwood's "The Willows" was one of the finest feats in weird literature, and I kept thinking back on it as Annihilation continued. In "The Willows," there are no stalking creatures threatening bodily harm, just a natural area where reality gives way to the unreal. That's not to say Annihilation lacks physical danger; wait until you discover the truth of the title. But aside from physical harm, Annihilation is the story of someone surrounded by presences and forces that may not be able to be understood, yet are surely advancing towards them. 
 VanderMeer made a great decision to make the main character a biologist, by the way. The way she tries to puzzle out Area X using ecology never failed to entertain, and often went past that right into disturbing. 
 The protagonist is up to the standards of the gripping narrative. We start to learn details about the biologist slowly, but the novel presents a very clear, comprehensive picture of her that never seems extraneous or less than carefully made. She breathes through these pages, pulling the reader forward with the force of her realism. 
 So, you have a wilderness that Lovecraft and Blackwood would be proud of, Kafka-esqe betrayal and conspiracy, a well-formed narrator and a climax that lives up to the premise. Read this. The moment I finished this, I ordered the next two in the trilogy, and I can't think of better praise than that.