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.
The first kill for Lich King heroic was at a 5% nerf, but the same guild did go back and kill it without the nerf later as well.
FL Rag is for sure in my top 5 for hardest bosses.
In no particular order.
Heroic Lei Shen
Heroic Spine of Deathwing
Heroic Lich King
Heroic Dark Animus