Many different answers have been posted on this subject. Instead of defining the answer for you though, I would like to offer something different: how to come to the conclusion for yourself.
Victor Frankenstein, from Mary Shelley's novel Frankenstein, creates a being (a monster) who goes on a murderous rampage given the alienation he faces from both society and his "father," Victor.
Therefore, based upon the text, do you believe that Victor is the greater monster portrayed in the text? What support do you have in making this statement?
For you, how is Victor a monster? How is the creature a monster?
My suggestion would be to create a list of what makes each of the characters a monster. After creating the list, see which character's list contains the most items. Based upon this, you will find which character you find to be the most monstrous.
Most likely, you will come up with one of the following:
1. Victor was the greater monster because he alienated his "son."
2. The creature is the greater monster because he was a murderer.