It's up to the user to decide what "worst" means. Take "/" to mean "or".
Wasn't easy to fit them in to 20 options but I tried to leave the likely most hated raids on their own line:
AQ40
Battle for Mount Hyjal
Trial of the Crusader
Dragon Soul
Hellfire Citadel, or more accurately, the raid that we should have gotten after HFC
Tomb of Sargeras
I didn't raid BfA
Sanctum of Domination
And for reference, separated out:
Molten Core (Onyxia / ZG)
Blackwing Lair
AQ40 (AQ20)
Naxxramas
Karazhan / Gruul / Magtheridon
Serpentshrine Cavern / The Eye of Tempest Keep
Battle for Mount Hyjal / Black Temple / Sunwell / (ZA)
Naxx 2.0 / Obsidian Sanctum / Eye of Eternity
Ulduar
(Grand) Trial of the Crusader
Icecrown Citadel / Ruby Sanctum
Blackwing Descent / Throne of the Four Winds / Bastion of Twilight
Firelands
Dragon Soul
Mogu'shan Vaults / Heart of Fear / Terrace of Endless Spring
Throne of Thunder
Siege of Orgrimmar
Blackrock Foundry / Highmaul
Hellfire Citadel
Emerald Nightmare / Trial of Valor
The Nighthold
Tomb of Sargeras
Antorus, The Burning Throne
Uldir /Battle of Dazar'alor / Crucible of Storms I dunno what is actually a tier in BfA
The Eternal Palace / Ny'alotha
Castle Nathria
Sanctum of Domination
Sepulcher of the First Ones