Who brought the concept of the Omnitrix in Ben 10?

The concept of Omnitrix was actually the brainchild of Steve E. Gordon. He also thought up the character of Ben and ten superheroes. He didn't include any aliens.

You'll see that he wasn't one of the four writers who created the series Ben 10 but they must be indebted to Steve's original ideas. Often a successful series stems from an idea earlier than the series creators.