What is edisons biggest failure?

Actually inventions are not a matter of failing or passing. The light bulb is one example. Many of the attempts made to make an incandescent bulb failed. The concept finally got to a sell-able state after many failures.

The concept of electric power that the Edison Company developed never took off either. They were convinced the best power source available was a direct current source. Nikola Tesla's concept of an alternating source is the one we use today. While not a matter of passing or failing, the idea simply never became marketable.