What is the strongest muscle in the animal kingdom?

Guessing it should be the flea's back legs, the equivalent power would allow us to leap the Empire State building.

That depends what you mean by "equivalent".
That oft-quoted factoid about the flea makes the very false assumption that if you scaled the insect up to human size, the height of its jump can be similarly scaled up.

A smaller animal has a smaller height to weight ratio.
As the height of an animal is multiplied by ten, the muscular cross-section would be multiplied by a hundred.
This sounds like it should work in the larger animals favour, but its volume, and hence its weight would be a thousand times larger. As a flea's leg isn't designed for this height to weight ratio, I'm not convinced it jump any higher than a human.

This article explains how so difficult it is to determine the strength of animal muscles that this often asked question has yet to be answered with certainty:

http://www.elasmo-research.org/education/topics/r_bites.htm

Besides, the flee's weight is also a part of what allows it to jump so high.

But then you have the jaw muscles of the great white shark, which could crush pretty much anything organic that comes in its way. Or the blue whale's flipper. I wouldn't want to get smacked in the head by one of those. (The blue whale's heart also has to be incredibly strong to support such a huge animal.)