Grendel's mother!! Most of the time in literature when there's a definite "bad guy", they are portrayed as a heartless beast - something so horrible that we can't relate to his/her/its actions as an antagonist. Grendel has all the makings of that traditional antagonist. Then, after his death, we see an attack by his mother seeking revenge. How perfect is that?!? All of a sudden, Grendel has a mother who is just as concerned about his well-being as the rest of our parents; somebody hurt her little boy so she needs to take action. To me, that added to the characterization of Grendel more than any of the Geats whose blood he drank and bones he crushed.
I would have to say that the quenn was my favorite character becasue she was pretty much the only one who didn't have ulterior motives in the whole story.