Constantine, for his spreading of Christianity.Which of the Roman Emperors do you think had the greatest influence upon western civilization? Please limit your thoughts to those who were officially called emperor? Julius Caesar was technically called "dictator-for-life" even though he had all the power of an emperor?
Augustus - without him, no Roman Empire. Without the Roman Empire, no Western civilisation as we know it.
Constantine comes second, but the changes we associate with him are more the joint result of several emperors' efforts.