Most useless villain.
Dr. Light. Not in terms of powers, mind you. But Arthur Light was just such a putz. He could manipulate light in various ways, which you would think could be very deadly in the wrong hands, but this guy somehow manages to screw that advantage up. He gets defeated by the Justice League, so he decides to go after easier prey in the Teen Titans and they defeat him (time and time again). So, powerful? Yes. But oh so powerless, and until Identity Crisis, a joke (ok, still somewhat of a joke).