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)