This is actually a much more complicated question than it first appears. The antagonist is defined as the character or force who is trying to prevent the protagonist achieving their goal. Obviously, Nora is the protagonist of this excellent play, but when we think about what she is trying to achieve, it becomes more complicated. It appears that Krogstad is the antagonist, as he is threatening Nora's security and position with the knowledge that he has. Thus we could argue that Nora's aim is to prevent Helmer discovering her fraud and to keep everything smoothed over in her marriage.
However, when we come to the final act of this play, and Helmer has discovered the truth, we come to reassess such conclusions. From what Nora says, we could state that Helmer has been the antagonist all along in the way that he has kept Nora in a "doll's house," just like her father, and treated her like a child rather than as an adult:
Now I look back on it, it's as if I've been living here like a pauper, from hand to mouth. I performed tricks for you, and you gave me food and drink. But that was how you wanted it. You and Papa have done m great wrong.
Thus, arguably, we can say that it is Helmer who is the true antagonist of the play, as he has constantly acted to prevent Nora developing a true self or understanding of her own identity.