For Whom the Bell Tolls is my favorite. Prose-wise, I like how Hemingway captured the formal Spanish translated back into stilted English, and style-wise, I think the way each character captures an "aspect" of why men (and women) fight wars is the essence of layered fiction.
"All good books are alike in that they are truer than if they really happened and after you are finished reading one you will feel that it all happened to you and afterwards it all belongs to you: the good and the bad, the ecstasy, the remorse and sorrow, the people and the places and how the weather was. If you can get so that you can give that to people, then you are a writer."
I think the guy accomplished exactly that when he got things right.
Some of his short stories are stellar as well.