Favourite Raid per Expansion, including Vanilla.

[Dakara]

1. Vanilla: Blackwing Lair
2. Burning Crusade: Black Temple
3. Wrath of the Lich King: Ulduar
4: Cataclysm: Firelands
5. Mists of Pandaria: Siege of Orgrimmar

[Deadlycurves]

Classic: Zul'Gurub
TBC: kara
Wrath: ulduar
Cata:Firelands
Mists: no longer raid so cant comment

[Haywire5714]

For all the people saying Siege of Orgrimmar ... forum policy dictates that you're not allowed to enjoy the current tier of content and acknowledge so publicly, instead you must let nostalgia guide you.

[Auberdeen]

Classic: Zul'Gurub
The Burning Crusade: Karazhan
Wrath of the Lich King: Ulduar
Cataclysm: Firelands
Mists of Pandaria: Siege of Orgrimmar

[Daetur]

1.) Naxxramas, though largely inaccessible to... well, everyone, was a big step towards more lore-driven raids, with a lot of interactivity as you progressed through- the Horsemen's yells, the noises from Thaddius... it was just "more" of the stuff they'd done before, and that was awesome. I also really, really liked the raid design with different wings.

2.) Honestly, I have to say Karazhan. It was incredible atmospheric. I want to say Sunwell, but I was really, really pissed with the lore and character direction in BC, and Sunwell was a great raid, but was soured by the weird ass Kil'Jaeden-crazy-Kael'thas plot and utter gutting of Kael, Vasjh, and Illidan's characterization from WC3.

3.) Hard choice. Ulduar was pretty damn near perfect, with some branching and wings, the implementation of hardmodes, and how damn creepy Yogg-Saron was. ICC was also an incredible, winged raid with an amazing endboss that really did justice to his lore.

4.) Firelands, in my opinion. Cata wasn't a great expansion for raiding, for me, nothing really gripped me in terms of theme or story. Firelands was decent, with Alysrazor and Staghelm and Ragnaros.

5.) Siege of Orgrimmar hands down. Absolutely incredible raid, ties together lore and story threads from across the entire expansion. Phenomenal. Could only be improved with wings.

[ringpriest]

Classic - BWL. (Never saw AQ/Naxx when they were current. I was a scrub.)
BC - Kara
Wrath - Ulduar
Cata - ICC (Descent had some fun fights, but I had more fun going back and helping people kill Arthas than I did with any Cata raid.)
MoP - no vote, didn't raid

[RoKPaNda]

Vanilla: Blackwing Lair. Naxx had the more entertaining encounters, but Blackwing Lair had better art for most things IMO.
TBC: Either Black Temple or Karazhan, everything else was meh.
WLK: ICC, again everything else was meh for me. Ulduar had too much trash and was large for the sake of being large.
Cata: Firelands.
MoP: Meh, probably SoO.

[Lighto73]

1. BWL, so awesome the first time.
2. Black Temple
3. ICC, didn't do Ulduar when it was current.
4. DS, I had fun, but mostly the guild I was in was best during Heroic DS.
5. SoO so far, missed out on heroics this expansion so I'm not 100% sure.

[kbyelol]

Classic - Didn't play
BC - Black Temple
Wrath - Ulduar
Cata - BoT
MoP - Not raiding anymore.

Tbh they were all fun the first 4-5 times. Then it just becomes boring and no fun at all.

[arioc]

1. Didnt Play
2. Kara
3. Ulduar with ICC as a very close second
4. First tier of raids
5. SoO

Also I didn't actually know anyone that seriously enjoyed ToC

[Ciddy]

I didn't raid in Classic (besides a couple of Onyxia pugs where I was like the 6th offtank) and haven't played enough in MoP to have an opinion there.

BC - Karazhan (Black Temple if Karazhan doesn't count due to being a 10man).
Wrath - Naxxramas. I know it was dumbed down and wasn't that long of an instance, but I loved the ambience and atmosphere of the place.
Cataclysm - BoT. Honestly, it was the only Cataclysm raid I really enjoyed.

[Ihnasir]

Molten Core, Black Temple, Ulduar, Blackwing Descent, Throne of Thunder

[alinaya]

1. Vanilla: Blackwing Lair
2. Burning Crusade: none, really.
3. Wrath of the Lich King: Ulduar
4: Cataclysm: Dragon Soul
5. Mists of Pandaria: Siege of Orgrimmar

[Alvito]

1. Didn't play yet
2. Hyjal, it was the first 25 man I was a raid lead for
3. H ToC without running multiple runs to practice and save attempts we were 1st on the server for many of the kills and the first to get to 0 deaths Anub.
4. Firelands none of them were all that amazing so this is kinda by default
5. Well MoP made me quit raiding so it is hard to pick anything with limited time in there SoO wins though.

[Arbs]

1. Vanilla: Blackwing Lair
2. Burning Crusade: Black Temple
3. Wrath of the Lich King: Icecrown Citedal
4: Cataclysm: Firelands
5. Mists of Pandaria: Throne of Thunder

[Deleted]

1. Vanilla: AQ40
2. Burning Crusade: SWP
3. Wrath of the Lich King: Ulduar
4: Cataclysm: Firelands
5. Mists of Pandaria: None

[Supakaiser]

Vanilla: Didn't play
BC: Didn't raid
WOTLK: Probably ICC (but it lasted way too long)
Cata: BoT/To4W/FL
MoP: SoO but ToT was cool too

[Toiran]

1. Vanilla: Didn't play
2. Burning Crusade: Sunwell
3. Wrath of the Lich King: Ulduar
4: Cataclysm: Didn't raid - would be Firelands by what I've experienced going back.
5. Mists of Pandaria: Siege of Orgrimmar

[Reyina]

Vanilla - I didn't play but I did go back to MC as a higher level and it was still fun.
BC - Karazhan. It was my first raiding and it was just amazing in there.
WotLK - Ulduar. It felt epic and the fights were fun, especially hardmodes.
Cata - Firelands, it was the last serious raiding I did and it was actually challenging and fun.
Mists - I've only done the first two LFR wings of SoO but I think I like it best.

[gemana]

Vanilla: MC
BC: Kara
WotLK: ICC
Cata: Firelands
Mists: Siege