The following is a list of the top 100 novels of the 20th century, as determined by Modern Library - Random House. The ones in bold are books I have read.
Ulysses, James Joyce
The Great Gatsby, F. Scott Fitzgerald
A Portrait of the Artist as a Young Man, James Joyce
Lolita, Vladimir Nabokov
Brave New World, Aldous Huxley
The Sound and the Fury, William Faulkner
Catch-22, Joseph Heller
Darkness at Noon, Arthur Koestler
Sons and Lovers, D.H. Lawrence
The Grapes of Wrath, John Steinbeck
Under the Volcano, Malcolm Lowry
The Way of All Flesh, Samuel Butler
1984, George Orwell
I, Claudius, Robert Graves
To the Lighthouse, Virginia Woolf
An American Tragedy, Theodore Dreiser
The Heart Is a Lonely Hunter, Carson McCullers
Slaughterhouse Five, Kurt Vonnegut
Invisible Man, Ralph Ellison
Native Son, Richard Wright
Henderson the Rain King, Saul Bellow
Appointment in Samarra, John O'Hara
U.S.A. (trilogy), John Dos Passos
Winesburg, Ohio, Sherwood Anderson
A Passage to India, E.M. Forster
The Wings of the Dove, Henry James
The Ambassadors, Henry James
Tender Is the Night, F. Scott Fitzgerald
Studs Lonigan trilogy, James T. Farrell
The Good Soldier, Ford Maddox Ford
Animal Farm, George Orwell
The Golden Bowl, Henry James
Sister Carrie, Theodore Dreiser
A Handful of Dust, Evelyn Waugh
As I Lay Dying, William Faulkner
All the King's Men, Robert Penn Warren
The Bridge of San Luis Rey, Thornton Wilder
Howards End, E.M. Forster
Go Tell It on the Mountain, James Baldwin
The Heart of the Matter, Graham Greene
The Lord of the Flies, William Golding
Deliverance, James Dickey
A Dance to the Music of Time (series), Anthony Powell
Point Counter Point, Aldous Huxley
The Sun Also Rises, Ernest Hemingway
The Secret Agent, Joseph Conrad
Nostromo, Joseph Conrad
The Rainbow, D.H. Lawrence
Women in Love, D.H. Lawrence
Tropic of Cancer, Henry Miller
The Naked and the Dead, Norman Mailer
Portnoy's Complaint, Philip Roth
Pale Fire, Vladimir Nabokov
Light in August, William Faulkner
On the Road, Jack Kerouac
The Maltese Falcon, Dashiell Hammett
Parade's End, Ford Maddox Ford
The Age of Innocence, Edith Wharton
Zuleika Dobson, Max Beerbohm
The Moviegoer, Walker Percy
Death Comes to the Archbishop, Willa Cather
From Here to Eternity, James Jones
The Wapshot Chronicles, John Cheever
The Catcher in the Rye, J.D. Salinger
A Clockwork Orange, Anthony Burgess
Of Human Bondage, W. Somerset Maugham
Heart of Darkness, Joseph Conrad
Main Street, Sinclair Lewis
The House of Mirth, Edith Wharton
The Alexandria Quartet, Lawrence Durrell
A High Wind in Jamaica, Richard Hughes
A House for Ms. Biswas, V.S. Naipaul
The Day of the Locust, Nathaniel West
A Farewell to Arms, Ernest Hemingway
Scoop, Evelyn Waugh
The Prime of Miss Jean Brodie, Muriel Spark
Finnegans Wake, James Joyce
Kim, Rudyard Kipling
A Room With a View, E.M. Forster
Brideshead Revisited, Evelyn Waugh
The Adventures of Augie March, Saul Bellow
Angle of Repose, Wallace Stegner
A Bend in the River, V.S. Naipaul
The Death of the Heart, Elizabeth Bowen
Lord Jim, Joseph Conrad
Ragtime, E.L. Doctorow
The Old Wives' Tale, Arnold Bennett
The Call of the Wild, Jack London
Loving, Henry Green
Midnight's Children, Salman Rushdie
Tobacco Road, Erskine Caldwell
Ironweed, William Kennedy
The Magus, John Fowles
Wide Sargasso Sea, Jean Rhys
Under the Net, Iris Murdoch
Sophie's Choice, William Styron
The Sheltering Sky, Paul Bowles
The Postman Always Rings Twice, James M. Cain
The Ginger Man, J.P. Donleavy
The Magnificent Ambersons, Booth Tarkington
This next list was compiled by students in the Radcliffe publishing course, and I tend to agree with some of its choices more than the previous list.
The Great Gatsby, F. Scott Fitzgerald
Catcher in the Rye, J.D. Salinger
The Grapes of Wrath, John Steinbeck
To Kill a Mockingbird, Harper Lee
The Color Purple, Alice Walker
Ulysses, James Joyce
Beloved, Toni Morrison
The Lord of the Flies, William Golding
1984, George Orwell
The Sound and the Fury, William Faulkner
Lolita, Vladmir Nabokov
Of Mice and Men, John Steinbeck
Charlotte's Web, E.B. White
A Portrait of the Artist as a Young Man, James Joyce
Catch-22, Joseph Heller
Brave New World, Aldous Huxley
Animal Farm, George Orwell
The Sun Also Rises, Ernest Hemingway
As I Lay Dying, William Faulkner
A Farewell to Arms, Ernest Hemingway
Heart of Darkness, Joseph Conrad
Winnie-the-Pooh, A.A. Milne
Their Eyes were Watching God, Zora Neale Hurston
Invisible Man, Ralph Ellison
Song of Solomon, Toni Morrison
Gone with the Wind, Margaret Mitchell
Native Son, Richard Wright
One Flew Over the Cuckoo's Nest, Ken Kesey
Slaughterhouse Five, Kurt Vonnegut
For Whom the Bell Tolls, Ernest Hemingway
On the Road, Jack Kerouac
The Old Man and the Sea, Ernest Hemingway
The Call of the Wild, Jack London
To the Lighthouse, Virginia Woolf
Portrait of a Lady, Henry James
Go Tell it on the Mountain, James Baldwin
The World According to Garp, John Irving
All the King's Men, Robert Penn Warren
A Room with a View, E.M. Forster
The Lord of the Rings, J.R.R. Tolkien
Schindler's List, Thomas Keneally
The Age of Innocence, Edith Wharton
The Fountainhead, Ayn Rand
Finnegan's Wake, James Joyce
The Jungle, Upton Sinclair
Mrs. Dalloway, Virginia Woolf
The Wonderful Wizard of Oz, Frank L. Baum
Lady Chatterley's Lover, D.H. Lawrence
A Clockwork Orange, Anthony Burgess
The Awakening, Kate Chopin
My Antonia, Willa Cather
Howard's End, E.M. Forster
In Cold Blood, Truman Capote
Franny and Zooey, J.D. Salinger
The Satanic Verses, Salman Rushdie
Jazz, Toni Morrison
Sophie's Choice, William Styron
Absalom, Absalom!, William Faulkner
Passage to India, E.M. Forster
Ethan Frome, Edith Wharton
A Good Man is Hard to Find, Flannery O'Connor
Tender is the Night, F. Scott Fitzgerald
Orlando, Virginia Woolf
Sons and Lovers, D.H. Lawrence
Bonfire of the Vanities, Thomas Wolfe
Cat's Cradle, Kurt Vonnegut
A Separate Peace, John Knowles
Light in August, William Faulkner
The Wings of the Dove, Henry James
Things Fall Apart, Chinua Achebe
Rebecca, Daphne du Maurier
A Hitchhiker's Guide to the Galaxy, Douglas Adams
Naked Lunch, William S. Burroughs
Brideshead Revisited, Evelyn Waugh
Women in Love, D.H. Lawrence
Look Homeward, Angel, Thomas Wolfe
In Our Time, Ernest Hemingway
The Autobiography of Alice B. Toklas, Gertrude Stein