Catherine the Great, née Sophie von Anhalt-Zerbst, started the process of changing the Romanov dynasty from Russian to German. Her successor, Pavel Petrovich, was fathered by her lover Sergei Saltykov. Pavel first married Natalia Alexeievna (Wilhelmina Louisa of Hesse-Darmstadt), then Maria Feodorovna (Sophie Dorothea of Württemberg). Subsequent Tsars also married German Princesses, except for Alexander III who married Princess Dagmar of Denmark who also took the Russian name Maria Feodorovna.
Having said that, Sophie/Catherine was the foreigner who made Russia great, a power to be reckoned with. Some of today’s conflicts trace to her reign - she acquired the Crimean peninsula and the Ukraine, Georgia, and the oil-rich Azerbaijan region. She was also involved in the partitioning of Poland.
She is known for continuing the reform efforts launched by Peter the Great and trying to bring Enlightenment ideas about government to Russia, an effort ended by the French Revolution. She attempted to reform education in Russia to make it widespread and accessible to both genders, a task that had to wait until long after her reign. She promoted the Russian Enlightenment in the arts and sciences. She also established what is now the State Hermitage Museum in St. Petersburg.
Her Wikipedia article is here.
