Accidents are by far the most common Origin.
Why? Because people are clumsy and stupid and fall into all sorts of radioactive/genetically engineered/acidic goo. The resulting Powers are usually quite epic in nature however they are as unpredictable and accident prone as their wielders.
Accidents are very cheerful and fun loving girls who will brighten up your Superteam’s Courage Pool however you must be prepared for them to fall on their arse: A lot!