Here are my favorite five:

1. The Great Gatsby
2. The Catcher in the Rye
3. Crime and Punishment
4. 1984
5. For Whom the Bell Tolls

Not necessarily in strict order:

1984 - George Orwell

One flew over the cuckoo's nest - Ken Kesey

One hundred years of solitude - Gabriel Garcia Marquez

War and peace - Leo Tolstoy

Le Grand Meaulnes - Alain Fournier

Wow, everyone is full of praise of Dostoyevsky's Crime and Punishment! I'll definitely start with it after I've finished One Hundred Years of Solitude (which I love completely, by the way).

And here are my favourite five, well, at least for the moment:

1. Daniel Deronda by George Eliot. Couldn't help but love it.
2. Anna Karenina by Tolstoy.
3. The Picture of Dorian Gray by Oscar Wilde. Marvellous.
4. Two Women (the original title is Twee Vrouwen) by Harry Mulisch, a Dutch author. I had to read it for school, but it is brilliant! Definitely worth reading, even if the translation is crappy (there are not many puns or wordplays (er, wordplays is literally translated from the Dutch woordspeling, it is a play with words, is it actually a word in English or is pun acceptable here?) in it, so it might be worth it.
5. Animal Farm by George Orwell. After reading it I started with 1984. I did not love 1984 half as much as Animal Farm though...

1984 - My favorite book. I haven't read it in quite some time.

The Jungle - The ending is basically a Socialist tract, and therefore, disappointing. I had a similar reaction to the epilogue of Crime and Punishment, which I nearly also listed as being one of the five anyway - too nuanced a novel for such a wild-eyed ending. But for reasons centering around an interest in animal issues The Jungle has provided me more cause for thought than has probably any other novel.

Foundation by Isaac Asimov. Written as an installment for a magazine this makes for one fine book. Violence is the last refuge of the incompetent. Some things just stick with you.

A Friend of Kafka by Isaac Bashevis Singer. My favorite author of (primarily) short stories. I picked this collection but it could have been The Spinoza of Market Street or even a stand-alone story, The Penitent.

Heart of Darkness by Joseph Conrad. For being the one work I was forced to read in school that I actually enjoyed it occupies this special place.

I think that Edgar Allen Poe should make the list, for any number of short stories, “The Tell Tale Heart”, or the “Pit and the Pendulum” are two that come to mind.

And for something completely different, George MacDonald Fraser, for his humor and satire.

The Brothers Karamazov
Moby Dick
The Lord of the Rings
Dracula
Pilgrim’s Progress

Trainspotting- Irvine Welsh
American Psycho- Bret Easton Ellis
The Wanting Seed- Anthony Burgess
Survivor- Chuck Palahniuk (he's overrated bigtime, but this book is still amazing)
IT- Stephen King


Yeah...lots of nihilistic pishposh in that list

Ooh I know this post is old but I'd like to post my favourites,

Anna Karenina -- Leo Tolstoy
Germinal -- Emile Zola
Roots -- Alex Haley
The Odyssey -- Homer
American Wife -- Curtis Sittenfeld

The Sheltering Sky - Paul Bowles

The House of Mirth - Edith Wharton

The Serval - Giuseppe Tomasi di Lampedusa

Decameron - Boccaccio

The Red and the Black - Stendhal

Honorary mention to Herman Melville's The Confidence-Man: His Masquerade, which is probably the most innovative and modernist work of 19th century American writing. I would have also had to give props to Germinal, but it was mentioned in the previous post.

My opinion on this changes all the time.

In no particular order...

The Grapes of Wrath - John Steinbeck
The Picture of Dorian Gray - Oscar Wilde
Atlas Shrugged - Ayn Rand
The Lord of the Rings (Yes, the whole series. Tolkien intended it to be one novel made up of six 'books')
The Stand - Stephen King (Still haven't finished this yet, but it's amazing)