I started raiding in BC, and can understand why people who did as well favor it more, but it honestly surprises me how many people favor BC. I don't disagree that it had quite a few great bosses, but it also had a considerable amount of extremely dull bosses as well. For every Vashj, KJ and Illidan there's at least 3-4 other bosses that bring it down in terms of quality in my opinion. I mean it's like people completely forget all of the bosses in the raids besides the last boss, like in Hyjal or The Eye. Karazhan was really the only shining raid of BC in my opinion outside of Sunwell.
WotLK as well seemed very lackluster outside of Ulduar and 2 bosses in Icecrown. It's just odd to me, and is as though people only focus on the great bosses while overlooking the entirety of the expansion. As well, I also think tier 11 is completely underrated and is equal to Ulduar in terms of quality of bosses. Ulduar is more favored due to it being a single raid and having more of a story behind it, but in terms of quality before Throne of Thunder, tier 11 is the only one even close to Ulduar. Firelands and Dragon Soul also had some very interestingly designed bosses as well.
MoP I have to leave out for now, as the final tier will likely decide it's placement, but if it's better than tier 14 (which wouldn't take too much) I'd likely place it first just ahead of Cata. So currently:
Cata > BC >= WotLK (BC and WotLK are very close imo)