Many to choose from. Here are three:
Louis XIV reigned in France from 1643-1715, 72 years, during which he rose France to one of the largest European powers, centralizing the government more on economic, trade and colonization, giving more power to the Crown than to the aristocracy or the Church. This was a move to a more modern State.
Peter I (Peter the Great) reigned from 1682 until 1725. He modernized Russia dramatically and quickly (by studying in Europe, which at the time was considered treasonous) and changed the country into a major international power by implementing progressive government reforms.
Charles I ruled England from 1625-1649. His reign ended with his execution, the English Civil War. In the aftermath, Oliver Cromwell would lead England in a short lived republican state. Charles’ impositions with taxation and his overriding Parliament led to the English Civil War, which led to the 10 years England would exist as a Commonwealth. Although the monarchy was restored in 1660, this series of events were major in the gradual transformation from absolute to constitutional monarchy.