At least, that's how Faulkner saw it, believing that The Sound and Fury, the first in his sequence of novels that took him far beyond what he'd previously achieved, was his greatest work. It's possible. Absalom! Absalom! is more dense and far-reaching, dredging even deeper pools of Southern misery; As I Lay Dying is the more sustained (albeit briefer) performance, the breathless work that has you in a chokehold from start to finish.
The Sound and the Fury, if it is stronger, is so because it's more humane. Much as I love As I Lay Dying, it's populated by grotesqueries, with only a few folks passed along the way emerging as humans we might care to know. Though Benjamin is "an idiot," Faulkner gives him a voice that's sensitive to human activity, and when we see him from the outside, the drooling child hunched at the table, we hardly recognize in his outward form the being we've come to know from within. Quentin's journey to suicide is broken up by scenes of foolishness, humor, decency, and confusion. Faulkner doesn't simply let darkness claim Quentin; rather, we see him choose his path, despite the liveliness of the world around him, a world in which he, to the last, participates. Jason is greedy and selfish, but Faulkner lets us see how difficult such a life is for Jason. He's never at peace, but constantly at odds with everyone; his vices give him no pleasure, leading him, I think, to mistake them for virtues. Lastly, there's Dilsey, who, though the focus of the final section, is never given a point of view position, and is even allowed to step offstage while the narrator waits for her to, say, retrieve an umbrella. It's clear that there's something of Faulkner's "race problem" in this: he can't get into the head of this character. His awareness of this, though, also makes him protective of her, moving him to elevate her. In the character descriptions in the appendix, written years later, she alone is given no textual comment. (I believe the sentence "They endured" does not refer to Dilsey but to all of the black characters—or, possibly, all of the characters. Dilsey is not a "they," and the way the text is set, below her name rather than beside it, indicates further that the descriptor is not meant for her alone. Why have no commentators mentioned this?)
The story's very end (prior to the appendix) felt a bit flat; something more needed to happen there, especially given how gloriously the prose takes off in Quentin's section. Otherwise, this is an astonishing work. I failed to read it in high school and had looked at sections of it over the years, but I'd never fully attempted it. Teaching Faulkner—and successfully taking on Absalom! Absalom! last summer—prepared me, so that I didn't find the novel difficult at all. (My one confusion, straightened out by a glance at a discussion of the novel, involved the presence of two characters named Quentin. I'm not sure Faulkner needed to do that.)