Bold claim, I know. But hear me out. While Naxxramas was the hardest vanilla raid, and will likely remain the hardest raid in WoW History, the Naxxramas most players experienced was in Wrath of the Lich King.
Naxxramas has some gorgeous art assets, and is steeped in lore. But few players got to experience it, and even fewer got to experience completing the raid. In Wrath, Naxxramas returned and became the first raid most players would complete. Because of this, more players got to experience Naxxramas and have FUN with it. That's worth a lot in my book.
The gang's all here.
Vanilla Naxxramas was the only way to get the Corrupted Ashbringer weapon.