Those lines that get under your skin and just sit there, heavy in your chest. I keep thinking about the part in Sylvia Plath's 'The Bell Jar' where Esther says, "I felt very still and very empty, the way the eye of a tornado must feel, moving dully along in the middle of the surrounding hullabaloo." It's not a loud, dramatic loneliness; it's that hollow, detached kind, where you're technically present but completely insulated from everything moving around you.
Another one that wrecks me is from Markus Zusak's 'The Book Thief'. Death narrating, "I am haunted by humans." It’s a loneliness born of eternal, unwanted observation, of being surrounded by life but never part of it. The loneliness isn't just in the sad person, it's in the entity forced to witness all the sadness and never truly share in the experience. It's profound in a really quiet, cosmic way.
For a more visceral, angry loneliness, I always go back to a line from 'The Song of Achilles': "I could recognize him by touch alone, by smell; I would know him blind, by the way his breaths came and his feet struck the earth. I would know him in death, at the end of the world." The loneliness is in the memory of a closeness so absolute that its absence isn't just an empty space, it's a whole world gone dark.