Without even looking at the other posts, my first response would be that they all were able to think "outside the box." No matter what was being done around them, and I'm certain they were aware, each was somehow able to offer something uniquely their own.
I doubt that most people would hesitate to connect the word "genius" to each of these men. My feeling, however, is not that it applies to how smart they were but to the kind of vision they had, and the courage they had to put themselves out in the public arena, regardless of whether what they shared was well-received or not. They each spoke "their piece," as only each could do. And while they were at it, they each changed the world.
Imagination is often something we think of with regard to storytellers and writers...artists. However, even though Shakespeare was a writer, he was so much more than a "storyteller." And Galileo and Einstein were able to use the flexibility of imagination to perceive what others were not able to. This is an interesting group of men to consider together.
Just about now, I also think I can't overlook Steve Jobs when I think of these men and this topic. What a legacy they all left behind: to change the face of the world in a positive way.