Mozart's Requiem -
up to the point that his student completed it after his death. I don't know if 'knowing' that Mozart didn't finish the music made me listen differently, but the last parts did feel unlike the earlier parts, perhaps more bombastic than he would have written.
but who knows, I could be imagining it.
I wasn't a huge fan of the Requiem when I first heard a portion of it, but enjoyed it more after I heard Colin Davis' rendition of it. For me, the recording really captures the visceral essence of the piece as a last testament.