My vote has to go towards zombies, through the years I had a blast killing millions of zombies on Resident Evil, Dead Rising, Left 4 Dead, etc.
You must be liking Shadow of the Colossus.Big huge honking creatures that you don't stand taller than their big toe. That's the most satisfying to me.
So little votes for humans? The most difficult opponent is one who is identical to you in form and ability, ie human. That is most satisfying.
Time.
How can that be when 98% of the time you are THE CHOSEN ONE