Wow, great topic. First, an aside to Englishman:
A Painted House - John Grisham.
Grisham -- no, no, he's rubbish, trash, a hack, aaaargh!
Captain Corelli's Mandolin - Louis de Bernieres.
An first-rate novel. Too bad Nicolas Cage is going to screw it up in the soon-to-be-released movie adaptation.
Okay, my list -- some of these helped with Witnesses and getting away from them -- all of them have helped me with getting along in life.
1) A Portrait of the Artist as a Young Man, James Joyce.
I get my handle from the main character, who escapes the net of family, religion, country, and devotes himself to art. I snuck out of a district convention when I was a teenager and bought this book ... it changed the way I read, thought, wrote.
2) Ulysses, James Joyce.
Yeah, it's difficult; yeah, you need a book of annotations to understand parts of it properly. But what most people miss is how funny this book is.
3) The Crying of Lot 49, Thomas Pynchon.
I can't say I've read Gravity's Rainbow yet, probably because it requires the same attention as Ulysses. This smaller and more accessible book is great for its uncanny depiction of American life, with all its lunacies, conspiracies, banalities ...
4) Selected Poems, James Tate.
A friend of mine had this book in high school; recently I got another copy, reread it, and the effect was ghostly, I guess. The poems to me are both new and familiar; I think it's more than nostalgia. Here's a poem, and you either get it, or you don't.
Frivolous Blind Death Child
The lake was filled with wax,
and the ducks were wax.
The reeds were wax reeds,
and the wind was a waxy wind.
How beautiful you are
asleep in my arms.
So as not to waken you
I gladly cut them off.
Frivolous blind death child
covered with stardust,
why do you stick your tongue
inside the clock,
why do you follow me
into the toolshed?
You have a third mouth
where most people hide
their third eye.
Can you taste the gold mountain,
can you taste the gold flower,
can you taste the gold knife?
No, no! That is the gold butterfly!
5) The Brothers Karamazov, Fyodor Dostoevsky.
The right of the son to murder his father; the right of man to dispense with God. Impossible to do justice here to this book.
6) Tom Jones, Henry Fielding.
Sometimes, when Joyce and Pynchon and Tate become irritating in their obscurity, it's refreshing to have a helpful narrator who does whatever he can to make your reading more pleasurable, even if it means taking time out from the story to ask how you are, share a friendly thought or two, fluff up your pillows, and warn you of what's to come.
7) Of Human Bondage, W. Somerset Maugham.
REM already mentioned this; I like it because Philip, the protagonist, discards all of his values, morals, and ethics (religious and otherwise) and starts over. But he doesn't lose his humanity -- he doesn't become a Raskolnikov -- doesn't murder his uncle when he has the chance.
8) If on a winter's night a traveler, Italo Calvino.
The first chapter is a seduction of the reader, who is the main character, who is trying to read a copy of If on a winter's night a traveler. This isn't an easy task, since the novel has been incorrectly bound by the publisher. So I suppose you both are and are not reading this novel when you read it. All comic postmodernist fiction should be this fun; where writers like Barth become tedious, Calvino stays fresh and interesting.
9) A History of the World in 10 1/2 Chapters, Julian Barnes.
Okay, so, this novel shouldn't be on a top ten list, but I'm putting it here anyway. It's comic, clever, sometimes a little cute -- in all, a pretty good anti-novel that is strangely obsessed with termites. A good amount of biblical satire. (A better choice for this spot on my list would have been Molloy, by Samuel Beckett, but it's too late to change anything now.)
10) One Hundred Years of Solitude, Gabriel Garcia Marquez.
Read this book. Read this book. Read this book.
That's it, more or less. I can't rightly call this my top ten, really -- just the ten books that first came to mind.