The Greatest is Superman. Dude can fly, super strong, x-ray vision, freeze breath, laser eyes, impervious to all attack except some substance that doesn't even exist naturally on Earth. Superman is like a GOD.
But I think the most interesting superhero is Spiderman. He's some young guy, a kid basically, who is trying to do good, yet gets vilified all the time. And despite his superpowers, he's always poor and struggling.