Actually it's probably rabbits, they take very little in terms of feed and water to get meat from especially if you go for a meat rabbit breed. Bonus points if you go for one who's fur is worth more, like the American Chinchilla vs the pure white ones that get a little bigger.

Though for maximum efficiency you really have to go for insects melee worms and crickets are up there in terms of protein conversion.

Whales are impractical like most carnivores since you have to raise their food, wasting plant matter to then feed the carnivore it's meals making it roughly twice as inefficient to eat a carnivore over an herbivore.

There is also the land use issue to get land use as minimal as possible you need to maximize calories/protein per acre and how possible that is will vary with the location. Some locations goats are ideal since traditional agriculture is impossible and they can forage effectively on it.

Also water constraints will limit what your best option is for maximizing the end goal, anyway I'm so far of track....

In a moral relativism sort of way, yes, but only if you consider the suffering of the animals. In practice, hunting whales creates ecological problems that have a deleterious effect on ocean life. Factory farming cows is also terrible for the environment, and I couldn't tell you which is objectively worse on the whole.

You could also argue that the level of awareness anf the capability to perceive pain should be considered. A whale's brain is theoretically higher functioning than a cow's, but we don't really understand neurology and consciousness among animals to say that a whale is more conscious than a cow. But we can say for sure that insects, for instance, do not have the same level consciousness as mammals. We could farm and consume crickets and be reasonably sure that the crickets neither perceive their predicament or suffer needlessly from the experience. So if you needed to kill a billion crickets to equate a single whale, which is worse?

Ultimately the distinctions we make are mostly arbitrary but practical. Cats are pets and cows are food. Why? Because someone decided that a long time ago, and it's easier that way. Cats are predators, and hard to grow for food. They kill pests (and songbirds, so keep your cats inside). Cows eat grass, something we can't eat, and turn that into steaks. Both animals are smart, both animals are cute, and both animals may form emotional bonds with humans.

I'm not sure about how economical and/or sustainable breeding blue whales for food would be