I hope this hasn’t been done before, I did a search and found nothing so here goes:
What do you think is the best opening line or lines to a book?
My favorite is:
V. S. Naipaul, A Bend in the River.: “The world is what it is; men who are nothing, who allow themselves to become nothing, have no place in it.”
Though I do have a soft spot in my heart for L’etranger; “Ma mere est mort”
The opening to Philip K. Dick’s Valis is fantastic. Unfortunately, my copy of Valis is in German, and the opening exposition is a bit long, so I’m hestitant to put any of it here, copyright and translation problems and all.
I’m partial to the following, mostly because it is the opening to one of my favorite books of all time, * Crossing to Safety * by Wallace Stegner:
“Floating upward through a confusion of dreams and memory, curving like a trout through the rings of previous risings, I surface. My eyes are open. I am awake.”
Oh, and I’d second * The Stranger * as a very haunting start to a great piece of literature.
Cannery Row has one of the most amazing openings I have ever read. It has perfect flow and is almost lyrical. Nabokovs Lolita has a very poetic first chapter and almost lulls you into a trance with the repetition of his muses name. Vonnegut usually has strong openings, and Sarte manages to make me nhilistic within in the first few pages, Nausea being the best example. My favorite opening sentance would be from Charles Bukowski’s Ham on Rye. “The first thing I remember is being under something.” For some reason that just does it for me.
“It was a dark and stormy night; the rain fell in torrents–except at occasional intervals, when it was checked by a violent gust of wind which swept up the streets (for it is in London that our scene lies), rattling along the housetops, and fiercely agitating the scanty flame of the lamps that struggled against the darkness.”
–Edward George Bulwer-Lytton, Paul Clifford (1830)
::: runs away :::
The first page of Snow Crash, which describes the Deliverator, his cargo, his weapons, and his car, is possibly the best set of opening sentences in all of SF.
“I hadn’t meant to kill the cat.”–Telempath, by Spider Robinson.
Jane Eyre: “There was no possibility of taking a walk that day.”
Sets the scene, the mood, essentially gives you the entire novel in a sentence. If only the weather had been decent, Jane could have taken her walk and not wound up married to a blind man with a crazy wife in the attic.
“In a hole in the ground there lived a hobbit.” - Tolkien
Honorable mention:
The beginning of Fear and Loathing in Las Vegas. The book is okay, but the summary of the the two primaries running around LA loading up on various “treats” is brilliant.
The opening of Alfred Bester’s The Stars my Destination – not the opening sentence, but the opening sequence – is a superb hook. I’d love to see it done – properly-- as a movie.
From “Life, the Universe and Everything” by Douglas Adams:
The regular early morning yell of horror was the sound of Arthur Dent waking up and suddenly remembering where he was."
I miss Douglas Adams 
“Harrison Begernon” - a short story by Kurt Vonnegut. Can’t remember the exact date but :
“It was the year 2035 and everyone was finally equal.”
Two come to mind - the most paranoid openings that I can think of 
William Burroughs - Naked Lunch
and Franz Kafka’s The Trial
There are ones that I think are better, but these are the ones that are most vivid and powerful for me personally.
"I am a sick man . . . I am a spiteful man. I am an unattractive man. I believe my liver is diseased. "
*Notes from Underground *
by Fyodor Mikhailovich Dostoevski
“The most merciful thing in the world, I think, is the inability of the human mind to correlate all its contents. We live in a placid island of ignorance in the midst of black seas of infinity, and it was not meant that we should voyage far. The sciences, each straining in its own direction, have hitherto harmed us little; but some day the piecing together of dissociated knowledge will open up such terrifying vistas of reality, and of our frightful position therein, that we shall either go mad from the revelation or flee from the deadly light into the peace and safety of a new dark age.”
H. P. Lovecraft, The Call of Cthulhu
(OK, it’s a short story, not a book. So sue me. ;))
HenrySpicer reminded me of the all time classic opening lines.
“When Gregor Samsa awoke one morning from unsettling dreams, he found himself changed in his bed into a monstrous vermin.” - Franz Karka, The Metamorphosis
The endnotes discussing this single sentence are several pages long.
“Take my camel, dear,” said Aunt Dot as she returned from High Mass.
Towers of Trebizond. Rose Macauley
[From memory, so may be slightly off.]
“The year was 2081, and everybody was finally equal. They weren’t only equal before God and the law. They were equal ever which way. Nobody was smarter than anybody else. Nobody was better looking that anybody else. Nobody was stronger or quicker than anybody else. All this equality was due to the 211th, 212th, and 213th Amendments to the Constitution and to the unceasing vigilance of agents of the United State Handicapper General.”
“In five years, the penis will be obsolete,” said the salesman.
- Steel Beach - John Varley
“Call me Ishmael.” - Moby Dick by Herman Melville
“Call me Jonah.” - Cat’s Cradle by Kurt Vonnegut