If Willy is the protagonist , who or what constitutes the antagonist force? Is the antagonist the same as if one considers Biff as the...

It would not be the same.

If Willy is the protagonist, the antagonist would be the force that wrecks him: The fake ideal of the American Dream and its assumption that hard work earns you the happiness that you deserve.

If Biff is considered the protagonist, his antagonist would have been his own father. He was the character who changed Biff's life and sent it spiraling down. It was his Dad's infidelity what made Biff change the image of greatness that he had of his father, and eventually what made his life lose meaning.