Strength is measured as strength per pound of body weight and pure brute strength. Therefore, the elephant is the strongest mammal on land, and the blue whale is the strongest animal in the sea. A gorilla is the strongest of all of the large mammals, pound for pound.
African elephants can carry almost 20,000 pounds. Gorillas can move over 4,000 pounds. The blue whale is the largest animal on the planet, and it resides in the ocean. Its tongue alone weighs more than an elephant, so it has to be the strongest mammal by far, even though it resides in the water.