The very mention of O. Henry reminds me of The Gift of the Magi, which chokes me up no matter many times I read or hear it. I would give anything to have someone love me the way Jim and Della love each other.
Shel Silverstein’s The Giving Tree is hard to read without getting all quavery.
And there are several places in Ben Okri’s The Famished Road where I weep - he’s got the most beautiful blessing for a newborn child, ever (I used it when I put together the “parents page” for my son’s high school graduation). And the last line of the book, which I am only paraphrasing here and may be mangling pretty badly: “Sometimes, the best part of life is only in a dream.”
Pft, that’s stolen from Calderón’s La vida es sueño 
(life is a dream).
“I pass the test”, she said. “I will diminish, and go into the West and remain Galadriel.”
Thank y’all for answering. I have some new reading material added to my kindle based on some of the descriptions here.
But it’s not the sentence on its own that creates the impact - it’s the weight of the book before it that makes it so moving. Besides, there is a huge difference between “life is a dream” and saying that the good parts of life are ONLY a dream.
Let me see if I can go find the book and write the actual sentence…Yeah, I screwed it up pretty bad. Here’s the real last sentence:
A dream can be the highest point of a life.
Another vote for the end of Charlotte’s Web.
Also this passage from The Velveteen Rabbit, which never fails:
If I make it through the rest, that last sentence gets me every time.
Terry Pratchett’s final book.
When Granny Weatherwax makes the rounds before dying, and you know its not truly her going around… but Terry saying goodbye to his beloved creations.