Karazhan: Nightbane
SSC: Lady Vash
Tempest Keep: Kael'thas
Mount Hyjal: Archimonde
Black Temple: The Illidari Council
Sunwell Plateau: -
Naxxramas: Was any boss hard in WotLK version of Naxxramas? I remember them all being cleared even for our guild within one night.
VoA:
Ulduar: Yog'saron
TotC: Anub'arak
Icecrown Citadel: The Lich King
Blackwing Descent: Nefarian
Throne of the Four Winds: Al'akir
Bastion of Twilight: Cho'gal
Firelands: Ragnaros
Dragon Soul: Spine of Deathwing
Mogu'Shan Vaults: Will of the Emperor
Heart of Fear: Grand Empress Shek'zeer
Terrace of Endless Spring: Sha of Fear
Throne of Thunder: Lei Shen
Siege of Orgrimmar: Garrosh
Highmaul: Imperator Mar'gok
Blackrock Foundry: Blackhand
Hellfire Citadel: Archimonde
Emerald Nightmare: Cenarius
Trial of Valor: Helya
Nighthold: Gul'dan
Tomb of Sargeras: Kil'jaeden
Antorus: Argus
Uldir: G'huun
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Not in every raid. I remember Illidan back in the Black Temple (for our guild) being easier for me than the Illidari Council fight, but for the majority of the raids you are right. It's definitely noticeable after the Mythic raid difficulty. In the older WoW, there were some exceptions, but nowadays the final boss is always the hardest, except when it comes to Xavius.