1- Onyxia40 (yes yes I loved Onyxia even it was a single boss, fond memories on the big turkey ^^)
2- Karazhan (for the layout and the atmosphere) I like Sunwell too but spent too few time there.
3- Ulduar (bosses where interesting and nice, and the raid itself was beautiful)
4- Firelands (loved the personnal responsibility there, and I found the bosses impressive)
5- Throne of Thunder (missed the beginning of SoO so I reserve my judgement on it)
1. Vanilla - didnt raid while current, but doing them later on, they were all garbage tbh.
2. TBC - Kara
3. WoTLK - Ulduar
4. Cata - BWD
5. MoP - hard choice, every tier in MoP was very good, but if I need to pick one then it's SoO for ammount of bosses and cool lore.
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.