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.

1742 Princess Sophie Friederike Auguste von Anhalt-Zerbst-Dornburg by Anna Rosina de Gasc (State Russian Museum - St. Petersburg, Russia)

ca. 1745 Grand Princess Catherine Alekseevna in hunting dress by Georg Christoph Grooth (Nizhniy Novgorod State Art Museum - Nizhniy Novgorod, Volga Federal District, Russia)

1745 or later Grand Princess Catherine Alexievna holding a fan by Georg Christoph Groot (location unknown to gogm)

1756 Grand Duke Peter Fedorovich, Grand Duchess Catherine Alexeyevna and a page by Anna Rosina de Gasc (National Museum - Stockholm, Sweden)

1762 (before) Grand Duchess Catherine Alexeyevna (1729-1796) by ? (Muzeum Narodowe w Warszawie - Warsawa, Poland)

1762 Grand Princess Catherine Alexeevna in mourning, possibly by V. Eriksen (State Russian Museum - St. Petersburg, Russia)

1762 Catherine II in coronation regalia by ? probably by or after Ericksen (location unknown to gogm)

A replica of one of Catherine the Great's dresses (Catherine Palace - Pushkin, Leningrad Oblast, Russia)

1762-1764 Catherine II in front of a mirror by Vigilius Ericksen (State Hermitage Museum - St. Petersburg, Russia)

ca. 1770 Miniature of Catherine the Great as a young woman after Georg Christoph Grooth (Hillwood Estate, Museum & Gardens - Washington, DC, USA)

1783 Catherine II as Legislator in the Temple of the Goddess of Justice by Dmitri Levitzky (Russian Museum, St. Petersbburg)
