I'll nominate
Heart of Darkness by Joseph Conrad. It's a short novel but incredibly powerful as a result. Here's an extract from Wikipedia:
"
Heart of Darkness (1899), by Joseph Conrad, is a short novel, presented as a frame narrative, about Charles Marlow’s job as an ivory transporter down an unnamed river in Africa. ... a mighty big river, that you could see on the map, resembling an immense snake uncoiled, with its head in the sea, its body at rest curving afar over a vast country, and its tail lost in the depths of the land. In the course of his commercial-agent work in Africa, the seaman Marlow becomes obsessed by Mr. Kurtz, an ivory-procurement agent, a man of established notoriety among the natives and the European colonials.
"The story is a thematic exploration of the savagery-versus-civilization relationship, and of the colonialism and the racism that make imperialism possible. Originally published as a three-part serial story, in ‘Blackwood’s Magazine’, the novella Heart of Darkness has been variously published and translated into many languages. In 1998, the Modern Library ranked Heart of Darkness as the sixty-seventh top-novel of the hundred-best-novels in English of the twentieth century; and is included to the Western canon."
There are moments in this novel--such as the appearance of the African woman--and those final words of Kurtz--which are unforgettable.
In 1979 it was made into a film
Apocalypse Now which changes the setting but keeps the basic thematic approach--but loses the intensity of the novel.
It is in the Public domain and easily available from PG and here at
https://www.mobileread.com/forums/showthread.php?t=17486