Final answer:
The strongest evidence against the defendant in Act 1 of 'Twelve Angry Men' is the testimony of the old man who claims to have heard the threat and seen the boy fleeing the scene.
Step-by-step explanation:
In the first act of Twelve Angry Men, the strongest piece of evidence against the defendant is the purported eyewitness testimony of the old man living beneath the apartment where the murder occurred. He claimed to have heard the boy say, āIām going to kill you,ā followed by the sound of a body falling, and then said he saw the boy running down the stairs after the crime. This evidence creates a direct link between the defendant and the crime scene, casting significant suspicion on him and making his involvement in the crime appear quite plausible.