I'd probably say that the hive have the most intelligence at an individual basis (upper echelons + witches), but I can't ignore that, as a race; thrall and ogres exist... even Knights are pretty much 'don't think, just kill' but they are a little more considered. The cabal are an in-game joke, which is quite ironic since they have been the biggest threat to the light in the d2 campaign (red war + lightfall). So that leaves the fallen.