I think people are attracted to extreme things in general and power in particular. The great white shark is great because it's huge and powerful, though it's considered monstrous in popular culture. In my experience it's also the favorite shark of nearly any kid who likes sharks, probably because it is immense and powerful.

"World's [blankiest] [blank]" is generally a pretty popular genre, regardless of the subject. Whether it's the tallest mountain, most venomous snake, longest railroad or fastest plane, people are fascinated with extremes. I think the obsession with the world's 'great' leaders like Genghis Khan or Alexander is just another manifestation of this.