Which critical/literary theory is best to interpret Heart of Darkness?

I think that one could make a strong case that Conrad's work becomes more powerful when examined from a Modernist point of view.  Given how the novel was published at the turn of the century, it becomes clear that Conrad's work is inserted into a progression of industrialization, imperialism, and growing advancement that reflects a certainty in how nations dominate and control other nations.  Marlowe is a part of this certainty and by the end of it, is transformed by it into being more ambiguous about it.

It is in this frame of reference where I think that a Modernist reading of the work illuminates so much from it. If one takes the Woolf definition of Modernism, Conrad's work embodies it in a strong manner:  "“All human relations shifted, and when human relations change there is at the same time a change in religion, conduct, politics, and literature.”  Conrad's work speaks to how there is a fundamental "shift" in human relations and how the work generates a "shift" in how relations are constructed.  The supposed certainty with which European nations colonized the world, in particular areas such as Asia and Africa, is "shifted" in Heart of Darkness.  When Kurtz says, "Kill the brutes" and then is reduced to a shell of his former self with his final words of "The Horror," it becomes clear that the novel indicates a shift in how Europeans and Americans were interacting with the world.  Through this, one can ascertain that Conrad is making a statement about how there are limits to Western imperialism, a frame of reference that shifts the understanding of the time period. The fact that the novel is told in a frame story where the narrator is also "shifted" helps to embrace another Modernist tendency.  Analyzing the work in a Modernist approach allows the individual to understand the modern sense of alienation that was so intrinsic to works of the time period.  There is a hollowness evident in the mission, in Kurtz, in Marlowe, and in the very ideas that individuals held to be true.  Part of the shift that Conrad is able to accomplish is to effectively question the "absolutes" of order and disorder, sanity and insanity, as well as how hypocrisy is a part of the Modern condition.  It is here in which analyzing the work from a Modernist frame of reference illuminates so much of its effectiveness, and thereby might be the best approach to take.

Which literary critical theories can be applied to Heart of Darkness?

Heart of Darkness is a novel that lends itself to many different literary theories due to its complex thematic exploration of race, gender, class, and colonialism.

One of the most commonly applied literary theories is a postcolonial reading, wherein some of the underlying colonial assumptions of the author are challenged through our modern understanding of colonization. For although Heart of Darkness condemns the capitalistic roots of colonialism through its portrayal of Kurtz, the native people of the Congo are still undeniably portrayed through a European lens. While they are afforded some humanity, there are elements of tribalism and primevalism that permeate Conrad's depiction.

Another literary theory that could be applied to Heart of Darkness is feminism. Kurtz's intended, although possessing a small narrative role, is a thematic linchpin of the novel. Marlow's decision to lie to her about Kurtz's final words casts the Intended as symbolic of all women who were actively misled about colonialism through propaganda and deliberate deceit. It is also interesting to examine intersectionality in the text, specifically the intersection between race and gender. The way in which Conrad portrays the native women of the Congo presents a stark contrast to his characterization of the Intended, and there is an underlying sexuality that permeates the prose describing the Congolese women, which is naturally not present when Conrad describes the Congolese men.

These are just two literary theories, but many more can be applied. The novel can be viewed through an autobiographical lens, since many of the events that happened to Marlow were directly inspired by Conrad's own life. Similarly, the imagery of Marlow pressing deeper down the river to the "heart of darkness" can be interpreted as a psychological journey towards the darkest part of his own heart.