Snape's death ... when he demanded Harry look at him, so as to look into his eyes and see her's one last time ... I just fell apart. Couldn't read for a bit after that.
I rarely reread books, and have never completely reread a HP book (I've read parts of the first two books aloud to my youngest child), but I may just reread this one. I was so eager to see how she ended the series, and I whipped through Deathly Hallows so quickly that I didn't even pick up on this detail.
Two scenes in this book had me reduced to a puddle of tears: Harry and Hermoine's visit to the graves of James and Lily, and Dobby's death. I could barely breathe when I read the part where Harry digs Dobby's grave.