I can't believe that this thread has been around so long and I never responded.

1. Superman (Strange visitor from another planet. It's the origin that the rest are measured against.)
2. Batman (Murdered parents and a bat flying through the window.)
3. Spider-Man (Radioactive spider bites a 3-dimensional, human hero.)
4. The Flash (Lightning and chemicals.)
5. The Hulk (a man risks his life to save a kid from the gamma bomb.)