In Animal Farm, which animals were not considered good workers by the other animals?

The question is interesting because of its implications. 
On one hand, some animals might not be considered good workers because they are voices
of dissent.  It is interesting to see how Orwell constructs a situation where animals
who don't blindly follow the pigs' instructions are immediately considered "bad." 
Orwell might be making a statement on the nature of government in that it easily derides
those who are not immediately locked in to what is being said.  Instead, they deemed as
"not good" in order to defray reflection and thought as to why these individuals do not
follow the party line and the Status Quo.  Certainly, animals like Benjamin and even
Clover might fall into this category.  Both of them understand what is being done and
can sense the manipulation of the animals.  Due to this, these animals are discarded,
their voices not validated.  Being considered a "not good worker" is a way in which the
pigs' control is consolidated because it does not examine individual voice, but rather
eliminates it.  In demonizing "the other," those in the position of power benefit by
continuing their own control.  It in this light that Benjamin and Clover could be
considered "not good workers" because of the fundamental challenge they pose to the
Status Quo.