This month marks the 20th anniversary of the publication of David Foster Wallace’s Infinite Jest which I believe is, and can give my reasons why, the best and most important novel of the 20th century. I wrote about the experience of reading it for the first time on the occasion of Wallace’s death in (oh my God) 1998:

It was probably [in the New York Times Book Review] that I read a notice for Infinite Jest, and something about it must have grabbed me I don't really know what, nothing in any conceivable description of the novel, and there's no real way to describe the novel, sounds like anything I'd be interested in and but so nonetheless I ended up with a doorstop paperback edition of this 1,079-page novel, David Foster Wallace's second. That must have been 1997.
And I remember vividly going home every night for months thereafter and poring over those thousand-plus pages, savouring each line of this seemingly endless story. I also remember vividly the night I finished it and feeling distinctly sad that I would no longer be going home and spending my evenings with it. Sad.
It was then, and remains now, my favourite novel of all time.
This new, happier occasion the anniversary has been nicely noted in The New York Times Book Review:

How is it, then, that “Infinite Jest” still feels so transcendentally, electrically alive? Theory 1: As a novel about an “entertainment” weaponized to enslave and destroy all who look upon it, “Infinite Jest” is the first great Internet novel.
To fully understand what Wallace was up to, the book bears being read, and reread, with Talmudic focus and devotion… That 20 years have gone by and we still do not agree what this novel means, or what exactly it was trying to say, despite saying (seemingly) everything about everything, is yet another perfect analogy for the Internet. Both are too big. Both contain too much. Both welcome you in. Both push you away.
Theory 2: “Infinite Jest” is a genuinely groundbreaking novel of language. Not even the masters of the high/low rhetorical register go higher more panoramically or lower more exuberantly than Wallace not Joyce, not Bellow, not Amis. Wallace spelunked the O.E.D. and fearlessly neologized, nouning verbs, verbing nouns, creating less a novel of language than a brand-new lexicographic reality.
“Infinite Jest” surpasses almost every novel written in the last century, maintaining a consistent and mind-boggling descriptive mastery. As John Jeremiah Sullivan wrote after Wallace’s death, “Here’s a thing that is hard to imagine: being so inventive a writer that when you die, the language is impoverished.”
Theory 4: “Infinite Jest” is unquestionably the novel of its generation. [Later] I realized how completely the book had rewired me… Most great prose writers make the real world seem realer it’s why we read great prose writers. But Wallace does something weirder, something more astounding: Even when you’re not reading him, he trains you to study the real world through the lens of his prose.
[Wallace]’d given us one novel of generational significance; surely he’d write the novel that helped us define what the next century would feel like. Our great loss is that he didn’t. His great gift is that the world remains as Wallaceian as ever… and now we’re all reading his unwritten books in our heads. You have borne us on your back a thousand times. For you, and the joyful, despairing “Infinite Jest,” we will roar forever amazed, forever sorrowful, forever grateful.
And D.T. Max, Wallace biographer, has this piece in The New Yorker.

“Infinite Jest” is a novel about the narcotic power of language a power so overwhelming that Wallace has to shred narrative into tiny strips to keep it under control. Stories nest within stories; experiences are fragmented and regroup; there are bad jokes and goofy science fiction (giant feral hamsters are marauding through Vermont). What he understood and what we understand as his plan was a revival of a kind of novel that had gone out of fashion, one in which the writer hugs his characters to himself, closing the ironic distance that writers like Salinger had carved into fiction’s bedrock.
What really propelled “Infinite Jest” into the culture were not the critics but a cohort of readers, many of them in their twenties. The first wave of enthusiasts were bewitched by the book’s pyrotechnics “It was DFW’s lexical genius; no one had really seen it since Pynchon,” Matt Bucher, who runs the Wallace-l Listserv, remembers more than Wallace’s ideas about redemption. But soon a different kind of reader emerged to spread the word, the intense celebrants who carried it like a totem, aided in their interpretation of a crafty, complex story by Wallace’s promise of deliverance. “Infinite Jest” owes its diffusion through the culture most of all to this group.