Favourite Raid per Expansion, including Vanilla.

[Dungeonravor]

1. Vanilla - Naxxramas
2. The Burning Crusade - Karazhan
3. Wrath of the Lich King - Icecrown Citadel
4. Cataclysm - Firelands
5. Mist of Pandaria - Siege of Orgrimmar

[Tekkommo]

Vanilla: BWL
TBC: BT
WotLK: ICC
Cata: TotFW
MoP: SoO

[Dangg]

1. Vanilla: Only did MC
2. Burning Crusade: Sunwell
3. Wrath of the Lich King: Ulduar
4: Cataclysm: Bastion of Twilight
5. Mists of Pandaria: Siege of Orgrimmar

[Alsierus]

Vanilla - BWL
BC - Black Temple/Kara (tied)
WOTLK - Ulduar/ICC (tied)
Cata - Firelands
MoP - SoO

[Fullgore]

1. Vanilla - Molten Core I'd love to say Naxx but I didn't play it at 60
2. TBC - Kara
3. WoTLK - Ulduar (Also loved Icc)
4. Cata - DS
5. MoP - Soo

[saucywench]

1. Kara - still the best imho, concubines and wizards chess
2. Naxx
3. ICC

[Achtalon]

MC
Kara
Ulduar
Did not play
ToT

Ulduar was the overall best, though H Lei Shen may be the best fight, at least that I've ever completed.

[Jimson]

Vanilla: Did not raid
BC: Kara (now that I am not farming it week in and week out), BT (because I never got there at level and it seemed so prestigious)
Wrath: ICC (now that I am not farming it week in and week out... seriously, 64 Marrowgar kills?)
Cata: I stopped raiding after T11 and honestly did not enjoy any of the 3 first tier raids
MoP: I've only done them in LFR (and SoO in Flex, partly) so it's hard to judge, because of the chopped-up wings... SoO is pretty cool.

[Touch My Bankai]

Vanilla: Naxxramas
The Burning Crusade: Karazhan
Wrath of the Lich King: Ulduar
Cataclysm: ehhh, Firelands
Mists of Pandaria: Siege of Orgrimmar

[Doomaz]

Vanilla: BWL
Burning Crusade: MH
Wrath of the Lich King: none of them where dat good, ICC i guess
Cataclysm: FL
Mists of Pandaria: SoO