Final answer:
Eleanor of Aquitaine was the most powerful ruler among Margaret of Anjou, Empress Matilda, Isabella of France, and Eleanor of Aquitaine.
Step-by-step explanation:
Out of Margaret of Anjou, Empress Matilda, Isabella of France, and Eleanor of Aquitaine, the most powerful ruler would be Eleanor of Aquitaine. She was the queen consort of both France and England, and she played a major role in the reigns of her sons Richard I and John in England. She was also a wealthy and influential woman in her own right, as she inherited the vast lands of Aquitaine.
The ranking of the other rulers would be as follows: Isabella of France was also a queen consort of England and played a role in the reign of her husband Edward II; Empress Matilda was a claimant to the English throne during the Anarchy period but faced significant challenges in asserting her authority; and Margaret of Anjou was the queen consort of England during the Wars of the Roses but ultimately lost power.