Two stand out. The first came from a fan who said he loved the book so much he had someone sculpt a statue of the protagonist. He also had one made and sent to me. It's about a foot tall, and beautiful. It looks exactly like I had the character pictured.
The second came from an editor. During the editing process, she wrote a note in the margin of the manuscript, and underlined a sentence. I read it, expecting it to ask a question, or point out an error. Instead, the margin note read, "This is why I became an editor."
Well, I liked the sentence, too, but coming from a professional editor, it meant something special, and I've never forgotten it.