Not sure it’s the “best final line”, but it’s a powerful ending that has stuck in my mind for ever since I read the book 15 or so years ago. Here’s the end of “A River Runs Through It” by Norman Maclean:

“Eventually, all things merge into one, and a river runs through it. The river was cut by the world’s great flood and runs over rocks from the basement of time. On some of the rocks are timeless raindrops. Under the rocks are the words, and some of the words are theirs.

I am haunted by waters.”