In Rainbows (2007)
In isolation this would too mawkish for my taste, but the music makes it a definitive requiem. A central, mournful piano progression is surrounded by sparse, fidgeting electronic percussion and shot through with Thom Yorke's thin, pained voice singing like the words are being pulled out of him. It's all very dry, very close and personal.
Not comfortable listening by any means but beautiful and wonderfully constructed. There is relatively little pop music that can genuinely be described as haunting, but this is one such track.