oh, my favorite's gotta be from Salt to the Sea by Ruta Sepetys:

"Guilt is a hunter." 

I'll post some more good ones later when I have time. 

I think it's Stargirl: "When I was little, my uncle Pete had a necktie with a porcupine painted on it."

but I also like The Scorpio Races: "It is the first day of November, and so, today, someone will die."

and Peter Pan: "All children, except one, grow up." 

It would probably be "There is nothing lonelier than a cat who has been loved, at least for a while, and then abandoned on the side of the road." from the Underneath. Great book. Read it.

Another one would be "I began in darkness, and my nose tells me everything I know." from a Wolf Called Wander. Another must read.

My favorite is from A series of unfortunate events- the end: 

If you have ever peeled an onion, then you know that the first thin, papery layer reveals another thin, papery layer, and that layer reveals another, and another, and before you know it you have hundreds of layers all over the kitchen table and thousands of tears in your eyes, sorry that you ever started peeling in the first place and wishing that you had left the onion alone to wither away on the pantry shelf while you went on with your life, even if that meant never again enjoying the complicated and overwhelming taste of this strange and bitter vegetable.

Runner ups are:

There once was a boy named Eustace Clarence Scrubb, and he almost deserved it. - Narnia-Voyage of the dawn treader

And: If you are interested in stories with happy endings, you would be better of reading some other book. - Again from A series of unfortunate events- The bad beginning 

@Admin, I posted a comment with a few favorite first sentences here, and it did appear on the thread. But then I posted another comment, and now both my comments have disappeared. Do you know what happened?

Well . . . after I posted the second one, its format was good, so I went back and deleted the first one with the funny spacing. I surmise what may have happened is that when I deleted the first one, the second also disappeared because you likely submitted it as a reply to the first one. I think if you had submitted it as a new comment on the thread it would have been OK. I'm so sorry. Please submit again if not too much trouble.

Admin

"Look, I didn't want to be a half-blood." - Percy Jackson, the Lightning Thief 

Jay: My favorite is "The journalists arrived before the coffin did." from Nevermoor by Jessica Townsend. Chaser likes "So, there I was, tied to an altar made of out-dated encyclopedias, about to get sacrificed to the dark powers by a cult of evil Librarians" from Alcatraz vs. The Evil Librarians by Brandon Sanderson.

Chaser: Excuse you! I can speak for myself.

~Chaser & Jay

"It was in the spring of the year 1241 that rumors began to travel along the highroad from Kiev in the land of Rus that the Tartars of the East were again on the march." The Trumpeter of Krakow, Eric P. Kelly

"The house and the garden were waiting." The Time Garden, Edward Eager

"In his own eyes he was small, quiet, and dangerous." I Will Adventure, Elizabeth Janet Gray

"His name was Eustace Clarence Scubb, and he almost deserved it." The Voyage of the Dawn Treader, C.S. Lewis

"The Law of the Jungle - which is by far the oldest law in the world - has arranged for almost every kind of accident that may befall the Jungle People, till now its code is as perfect as time and custom can make it." The Second Jungle Book, Rudyard Kipling

Here they are again!

"This is a story of an adventure that happened in Narnia and Calormen and the lands between, in the Golden Age when Peter was High King in Narnia and his brother and his two sisters were King and Queens under him." -C. S. Lewis, The Horse and His Boy

"From the very beginning there was not the slightest doubt that Olga da Polga was the sort of the guinea-pig who would go places." -Michael Bond, The Tales of Olga da Polga

"If you want to find Cherry-Tree Lane all you need to do is ask the Policeman at the cross-roads." -P. L. Travers, Mary Poppins

"Long, long ago, when people spoke languages quite different from our own, many fine, big cities already existed in the sunny lands of the world." -Michael Ende, Momo

"The end of the world started when a pegesus landed on the hood of my car."

~The Last Olympian