Gotta disagree. I never beat heroic Rag but I have heard it is top 5 hardest. It was beat, though. On the other hand Lich King Heroic was never killed. No one ever did it before it was nerfed. I think that alone makes it the hardest wow boss ever, especially considering it was the height of popularity.
C'thun adn Yogg were supposedly impossible before the first nerf, too.
edit: wow was reading up on heroic rag. apparently 500 wipes was the fastest...maybe it is up there lol