Do you think Hamlet is at his worst in scene 3 or 4 of Act III and why?
Do I think he's at his worst here? No, neither, from my perspective. That would happen elsewhere in the play.
However, if we're only choosing between those two scenes, we have to define what we mean by "worst." He waffles more in scene 3—but he kills the wrong guy in scene 4. Since waffling (sitting around and trying to decide) has been his great weakness in the play, I'd say that he is worse in scene 3. He has a chance to kill Claudius, but lets him live! As a result, he can make the mistake he does in scene 4. That makes scene 3 worse.