The Pax Romana could be considered the high point of the Roman Empire.  This lasted from the end of the republican civil wars and began when Augustus took power in 27 B.C.  It lasted through the death of Marcus Aurelius in 180 A.D.  What made it a high point was the fact that the Roman legal system brought order to the Roman provinces.  The borders of the Empire were successfully patrolled and the internal empire was free from any major foreign invasion or social disorder.  There were no real power disputes during this time. The internal order and peace during this time allowed the arts and architecture to thrive along with trade and the economy of the Empire.

I would say that the high point or golden age of the Roman Empire was very early in the actual Imperial period -- during the reign of the Emperor Augustus Caesar.  During this time, Rome was becoming more and more powerful in military terms and it was undergoing something of a golden age in terms of culture.  Augustus was the one, after all, who said that he found Rome as a city of brick and left it as a city of marble.

Augustus's reign was a time when Rome was strong and when it was enjoying a fairly good period in terms of the kind of government it had and in terms of its material culture.  These things combine to make his reign a high point in Roman history.