Final answer:
Constantine was the ruler most responsible for ending the persecution of Christians in Ancient Rome.
Step-by-step explanation:
The ruler most responsible for ending the persecution of Christians in Ancient Rome was Constantine.
Constantine ruled until his death in 337 and played a significant role in ending the persecution of Christians. He attributed his victory in 312 to the Christian God and issued the Edict of Milan in 313, which outlined a policy of religious toleration. This edict made Christianity no longer illegal and allowed most traditional Roman religious practices to continue.