Interestingly, based on what I'm seeing here, the mega-adventures seem to have been well-recieved -- they're right up there with the top player option and monster-type books, which means that they're doing better than the environment, FR, and Eberron lines, and better than books like DMG2 and MM4.
I'll probably vote for Complete Mage or Slaughtergarde this year. CM was good but suffers from the 'hey, -another- Complete Magicuser Book' syndrome, and Slaughtergarde I really just don't know as well as the others, and I really like all the others.