Tesla
It's not easy to separate the true story from the myth. Tesla did a lot of real work and research, but also did a lot of "so far out of the box you can't even see the box from here" thinking.
While Tesla was working on ways to transmit power through the air, nothing he did in real-world work (to the best of my knowledge) promised "free energy."
In my book, one of many great men and women whose real contributions outweighed their looney factor.