Favourite Raid per Expansion, including Vanilla.

[Rioo]

Vanilla - Molten Core
TBC - Karazhan
WotLK - Ulduar
Cataclysm - BWD
Mist - SoO

[Chickat]

Vanilla - Molten Core
tbc - Tempest Keep
Wrath - Ulduar
Cata - Firelands
MoP SoO

[Haywire5714]

Vanilla - AQ40
TBC - Black Temple
Wrath - ICC
Cataclysm - Firelands
MoP - Throne of Thunder

[Iceman2500]

Vanilla - Zul'Gurub
BC - Karazhan
Wrath - Ulduar, also my top favorite raid in the entire game.
Cataclysm - Blackwing Descent; Nefarian's presence really made it stand out to me.
Mists - SoO; the other tiers are good, but Siege makes me feel like I'm back in the Warcraft universe again.

[Grimbold21]

I know, i know, they're just opinions, but did the OP just say ToC as his favorite wotlk raid?

Vanilla - MC
TBC - All of them! It's freakin TBC!!!!
Wotlk - Meh ICC i guess
Cata - hardly raided it
Mists - see above

[Designate]

Didn't start til Cata so,
Cata: Bastion of Twilight
Mists: Heart of Fear

[Grand Crusader Absalom]

Vanilla: ZG
BC: Sunwell, although Kael'thas is my favourite encounter of all time.
WotLK: ICC or ToC - I fucken hated Ulduar
Cata: DS
MoP: SoO

[scarson]

I know, i know, they're just opinions, but did the OP just say ToC as his favorite wotlk raid?

Vanilla - MC
TBC - All of them! It's freakin TBC!!!!
Wotlk - Meh ICC i guess
Cata - hardly raided it
Mists - see above

Same, with the exception of Ulduar for WotLK

[anon5123]

Vanilla: Didn't play during Vanilla, but BWL seemed like a cool raid.
BC: Only raid I did in BC was Kara, and that was pretty damn cool.
WOTLK: Ulduar without a doubt.
Cata: It's a tie between Firelands and BoT/BWL. I liked Firelands' outdoors-y, non-linear feel. Gameplay wise though, I liked T11 better.
MoP: Haven't completed any raids in MoP yet, only a few random LFR sections.

[QwertySham]

1. Didn't play
2. Didn't play
3. ICC (Quit before Ulduar came out, and came back when ToC came out -.- )
4. Firelands
5. Loving SoO right now. Might be biased because all raids leading up to SoO were all done in LFR. Now I'm doing flex, and enjoying it much more.

[Deleted]

1. didn't play
2. Zul'Aman
3. Ulduar
4. Firelands
5. Siege of Orgrimmar

[Zatetic]

MC
Kara
Ulduar
BWD
SoO

[slanik]

Vanilla: Naxx
Bc: Sunwell
Wrath: Icc
Cata: Firelands
Mists SoO

[Ragnarocket]

Vanilla: Wasn't able to play in raids at the time, but I'll go with AQ40. Very epic feel to it.
Burning Crusade: Karazhan for 10s/Sunwell for 25s
Wrath of the Lich King: Ulduar hands down. ICC close second.
Cataclysm: Not really a fan of any honestly...I'd go with Blackwing Descent though.
Mists of Pandaria: Only played for the first raid tier, loved Heart of Fear though.

[HeedmySpeed]

1. Temple of Ahn'Qiraj***
2. N/A*
3. Ulduar***
4. Blackwing Lair
5. N/A*

***didn't experience raids when content was current
*haven't completed all raids during expansion

[Dom]

Uld
BoT
MSV

[Dilbon]

Zul'Gurub
Karazhan
Naxxramas

[ItsInTheAir]

Vanilla: MC
BC: Kara
Wrath: Naxx
Cata: Didn't raid
MoP: Don't play

[melodramocracy]

1) wasn't around
2) wasn't around, in retrospect it probably would have been Kara
3) Ulduar
4) Firelands
5) SoO or MSV

[Aaros]

My fav raids by expansion:

Classic: Did not play, but I did do a couple of the raids on my old (first) toon when he was level 63: MC and BWL. Of the two, I preferred Blackwing Lair.

Burning Desire for Attune--- err... Burning Crusade: Did not have the privilege of playing during this time, but I do remember doing a run of the Sunwell Plateau on my 85 rogue during Cata. It was on the KJ server, too. Imagine that. EDIT - Honorable Mention: Serpentshrine Cavern. I remember doing this on an old toon when he was 70. It was during Wrath, but I haven't picked up the xpac, yet, at the time.

Wrath of the Lich Bi--- err... King: I started playing WoW during this time. I've heard stories of the great city of Ulduar, but to confess, I haven't actually set foot in the place, yet.

Cataclysm: I did a good bit of raiding here, especially on my aforementioned rogue. I'd say the Firelands was my favorite place.

Mists of Pandaria: Even though the setting here seems out of place, I'd say Mists is my most favorite expansion that I've played. Did the most raiding here (LFR, but raiding). I'd say the bastion of lightning that is the Throne of Thunder would be my favorite raid so far. I haven't tried the Siege, yet, because I wanna clear Throne first and get some better gear (7/12 LFR, 506/510 iLvl). Honorable Mention: Mogu'shan Vaults - Gives off an echo of Karazhan with a taste of Ulduar.