what do you think is the hardest fight in each raid?

[Kief]

Molten Core: Ragnaros
Blackwing Lair: Nefarian
AQ20: Ossirian
AQ40: C'thuun
Naxx: Kel'thuzad

Karazhan: Nightbane
SSC: Vashj
Tempest Keep: Kael'thas
Mount Hyjal: Archimonde
Black Temple: Illidan
Sunwell Plateau: M'uru

Naxxramas: Kel'thuzad
VoA: Lightning
Ulduar: Yogg-Saron
TotC: Anub'arak
Icecrown Citadel: Lich King

Blackwing Descent: Nefarian
Throne of the Four Winds: Al'Akir
Bastion of Twilight: Sinestra
Firelands: Ragnaros
Dragon Soul: Spine

Mogu'Shan Vaults: Will
Heart of Fear: Amber-Shaper
Terrace of Endless Spring: Sha
Throne of Thunder: Lei Shen
Siege of Orgrimmar: Klaxxi

Highmaul: Imperator (mechanically harder, Butcher was the overall hardest boss to down though)
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

I think the question is fundamentally flawed. There are almost no raid instances in the games history where the final boss was not the most difficult one. You can get into the argument of what constitutes difficulty, but sheer mechanical complexity is what constitutes difficulty imo. I don't think there is much room for changing much from the list I have made, if you look at objective difficulty.

[Xkiller9000]

Im not gonna add obvious ones like Yogg 0 light, all of these are hte base highest difficulty likeh eroic
Ulduar: Algalon
ToC: Anub'Arak
ICC: Lich King 25H
T11: Nefarian 25H
Firelands: Ragnaros25H
Dragon Soul: Spine of Deathwing heroic
T14: Elegon Heroic
ToT: Ra-den or Lei Shen heroic
SoO: Dark Shamans
HM: Imperator
BRF: Blast Furnace
HFC: Mannoroth/Archimonde
EN: Ilgynoth i think
NH: Star Augur
ToS: Fallen Avatar/KilJaeden
ABT: Argus

BFA
Uldir: Ghuun for sure (i didnt do the world first race, if i did, Fetid might be up here but)

[wulzurebrah]

Im not gonna add obvious ones like Yogg 0 light, all of these are hte base highest difficulty likeh eroic
Ulduar: Algalon
ToC: Anub'Arak
ICC: Lich King 25H
T11: Nefarian 25H
Firelands: Ragnaros25H
Dragon Soul: Spine of Deathwing heroic
T14: Elegon Heroic
ToT: Ra-den or Lei Shen heroic
SoO: Dark Shamans
HM: Imperator
BRF: Blast Furnace
HFC: Mannoroth/Archimonde
EN: Ilgynoth i think
NH: Star Augur
ToS: Fallen Avatar/KilJaeden
ABT: Argus

BFA
Uldir: Ghuun for sure (i didnt do the world first race, if i did, Fetid might be up here but)

Elegon was like one of the easier bosses in msv and t14 (on heroic, he is the hardest boss on normal msv though)
And are you serious when you say that dark shamans was the hardest boss you did in SoO?

[Purpleisbetter]

There should be a 10n and 25m separation in some cases.

ICC: LK, hands down

BWD: Neffy; early Omnotron
BoT: Chogalito
To4T: Al'akir, lol at the first one
FL: Raggy 25, which was one of the toughest for a long time. Paragons' number of wipes speak volume. Ryolith 10m was there, unless Arcane Mages with Sinestra trinkets
DS: Spine 10m, followed by Spine.

MV: WoE both sizes.
HoF: I think Vizier 10m > Empress 25, But that's maybe just me
ToES: Sha of F...rustration should be the undisputed king of whole T14 but did anyone ever enjoied Lei Shi?
ToT: Horridon pre nerf. Like really. Animus, Twins pre nerfs. Lei Shen pre nerfs. Ok ToT was one of the best raid ever but it was a Q&A shtshow. 10M was just a long frustrating run for the first raid lock outs
SoO: Siegebreaker 10 vs Klaxxi 25. Both equally badly designed, both were pretty much the end of their respective sizes. Garrosh was a joke compared to it.

BRF: tough raid but oh my Blackhand...
HFC: Archimonde i guess. Dunno tho, i struggled more on Socretar but eventually the rings kicked in with all its OPness...

EN: Xavius, really.
ToV: Q&A team doing a worse job than the one made for ToT. I eventually skipped the whole thing.
NH: Guldan is up there with Ely with the latter maybe a bit harder. But it was a very late progress, i dunno.
ToS: Avatar. Well i thought Blizzy could never ever top what they did with ToV but i was mistaken.
Antorus: Aggramar or Council. We sucked like always and we progressed through both with some fat overgearing. More than your average overgearing. Literally titanforge galore. We had like 5 TF 970+ Arcanocrystals dudes (i was one)and a plethora of FoTM rerolls.

[Moggie]

Molten Core: Geddon
Blackwing Lair: Chrommagus
AQ20: Ossirian
AQ40: Huhuran, least she was worse than Twin Emps for guild at the time.

Karazhan: Netherspite
SSC: Vashj
Tempest Keep: Kael
Mount Hyjal: Archimonde
Black Temple: Mother
Sunwell Plateau: M’uru

Naxxramas: N/A (nothing difficult there)
VoA: N/A
Ulduar: Yogg0
ToGC: Valkyr
Icecrown Citadel: LK

Blackwing Descent: Maloriak
Throne of the Four Winds: Really?
Bastion of Twilight: Chogall
Firelands: Spider boss
Dragon Soul: Spine

Mogu'Shan Vaults: Emps
Heart of Fear: Amber Shaper
Terrace of Endless Spring: Lei She
Throne of Thunder: Lei Shen
Siege of Orgrimmar: Paragon

Highmaul: Koragh
Blackrock Foundry: Blast Furnace
Hellfire Citadel: Archimonde

Emerald Nightmare: N/A
Trial of Valor: Odyn
Nighthold: Botanist or Elisande...depend whom you ask in my guild.
Tomb of Sargeras: KJ
Antorus: Argus

Uldir: Attendance boss.

[Tojara]

Karazhan: Shade of Aran and Netherspite felt way more difficult than Prince
Zul'Aman: Hex Lord Malacrass
Sunwell: M'uru was about the same, or more difficult than KJ

We raided 10 man, but for the most part Cataclysm was pretty standard, at least after the initial tuning was done at the start of Cataclysm (which was awful).

Bastion of Twilight: Cho'gall, Sinestra and Ascedent Council were all relatively the same difficulty
Dragon Soul: Spine of Deathwing was much more difficult than Madness of Deathwing

Mogu'shan Vaults: Spirit King's
Heart of Fear: Imperial Vizier Zor'Lok
Throne of Thunder: Megaera and Lei Shen
Siege of Orgrimmar: Siegecrafter was about the same difficulty as Garrosh

The Emerald Nightmare: Cenarius

Entirely based on my experience with the game. Two expansions I raided solely 10 man, but every expansion I experienced on the highest difficulty available. For the most part, sans a few instances, the last boss is either the hardest, or just as hard as another boss in the instance. The above is based on pure pull counts, and probably is probably pretty close to what most guilds experienced. I almost listed HFC as well, because Archimonde and Mannoroth were pretty close in difficulty by the time a lot of guilds got to him. Naxx in WoTLK makes me laugh whenever it's brought up, and I legit can't think of one boss being harder than any other (maybe Thaddius?). Ulduar would also be a special note because it depends on what you consider the final boss. Technically Algalon is the 'final' boss, but Yogg with zero lights is the most difficult.

[Etoo]

Elevator bosses in every raid.

[MrLachyG]

snip

only really started raiding in MoP so will start there. gonna try and avoid final bosses as they are usually harder anyway

Siege of Orgrimmar: Kor'kron Dark Shaman - oh good lord that fight was insane

Highmaul: while Imperator was difficult, Tectus could also be a pain in the ass
Blackrock Foundry: Blast Furnace - nothing else compared
Hellfire Citadel: Xhul - fuck that fight

Emerald Nightmare: Il'gynoth
Trial of Valor: Helya
Nighthold: Botanist
Tomb of Sargeras: Sasszine
Antorus: Coven of Shivarra

Uldir: Zul, Reborn

[Yunaqt]

only really started raiding in MoP so will start there. gonna try and avoid final bosses as they are usually harder anyway

Siege of Orgrimmar: Kor'kron Dark Shaman - oh good lord that fight was insane

Highmaul: while Imperator was difficult, Tectus could also be a pain in the ass
Blackrock Foundry: Blast Furnace - nothing else compared
Hellfire Citadel: Xhul - fuck that fight

Emerald Nightmare: Il'gynoth
Trial of Valor: Helya
Nighthold: Botanist
Tomb of Sargeras: Sasszine
Antorus: Coven of Shivarra

Uldir: Zul, Reborn

We killed Kor'kron first week alongside the others felt like a joke from my opinion, avoiding the final boss I would say Paragons was one of the hardest for SoO.

- - - Updated - - -

Started here

Karazhan: Nightbane
SSC: Vashj
Tempest Keep: Kael'thas - Imo the hardest of the expac
Mount Hyjal: Archimonde
Black Temple: Illidan

Naxxramas: Kt I guess lol
VoA: n/a
Ulduar: Yogg
TotC: Anub'arak
Icecrown Citadel: Lich King (25m) - The hardest of the expac by far

Blackwing Descent: Nefarion
Throne of the Four Winds: Al'akir, swimming mechanics are horrible
Bastion of Twilight: Ascendant Council
Firelands: Ragnaros - hardest of the expac
Dragon Soul: Spine of Deathwing

Mogu'Shan Vaults: Emperor
Heart of Fear: Shek'zer
Terrace of Endless Spring: Sha of Fear
Throne of Thunder: Lei Shen - I think Lei Shen was fractionally harder than SoO, but were great fights
Siege of Orgrimmar: Garrosh

Highmaul: Emporer Margok - For me, did this one second week so felt brutal at the time
Blackrock Foundry: Blackhand
Hellfire Citadel: Archimonde

Emerald Nightmare: Cenarius I guess
Trial of Valor: Hell'Yeah
Nighthold: Guldan
Tomb of Sargeras: Kil'jaeden - Hardest of the tier
Antorus: Argus

Uldir: G'huun

[Jastall]

Going with the raids where I raided at high levels as a Warrior (old Heroic, new Mythic), forgoing final bosses as these are usually the hardest anyway;

SoO: Blackfuse.

Highmaul: Butcher
BRF: Blast Furnace
HFC: Mannoroth

EN: Cenarius
ToV: Guarm
Nighthold: Star Augur
Tomb: Avatar of Sargeras
Antorus: Coven

Unsurprisingly, most of these are super melee unfriendly of course.