In cata so far, BWD and firelands are my favourite.
Tot4w was interesting, but only 2 bosses and al'akir was a pushover.
BoT is a little short and I don't know, it's missing something, could be that it is so linear, whereas the other raids are far bigger and you can choose where to go when you want etc.
BWD is awesome, great spread of bosses with some interesting mechanics. Nef fight was tough with all the rng pre-4.2 (guild cleared 12/12 normal and a few heroics before the nerf).
Firelands is very cool, and we have been working on rag for about 4 weeks now.

