If we define the antagonist as the character that prevents the protagonist from achieving her goals, there are six of them in the Wife's Prologue. The first is the Wife of Bath herself: she's her own worst enemy when it comes to making her point. Her mind has a tendency to wander, so that almost immediately after proposing to speak about the woe in marriage she launches into a defense of the serial marriage lifestyle. When she proposes to speak about her fourth husband, she wanders into a recollection of herself in her youth, which is probably due to self-centeredness as much as a wandering mind and mouth. The Wife's digressions are certainly entertaining, but they do compromise her authority quite a bit, preventing her from appearing as rhetorically polished as she'd like to.
The other five antagonists in the Wife of Bath's Prologue are the Wife's five husbands. She is explicit about how she is in competition with them to get the upper hand in the relationship, devoting a good portion of the Prologue to explaining how she managed to accomplish this. It seems that her first three husband were not much of a challenge for the Wife.
The Wife's fifth husband, Jankyn, presents more of a challenge for her; he takes control of the Wife's property, reproves her of her flaws, and forces her to listen to him read from a book of derogatory stories about women. The Wife cannot tolerate such a lack of control and respect in her relationship and, in the conflict that follows in which she and Jankyn come to blows, she claims to have regained all power in the relationship.
To speak more generally, any husband would be an antagonist for a woman like the Wife of Bath because a husband in medieval England was supposed to rule over his wife and household in the same way the king ruled over his lands and people. For a woman like the Wife, who asserts that women "love no man that taketh kepe or charge / Wher that we goon; we wol ben at oure large" (321-322), any husband represents an obstacle in her path and an antagonist to be overcome.