I know this sounds wierd but my favorite was a mutant origin (so I wimped out hehe) who could control and manipulate dangerous radiation. He could fly, forcefield, and release a terrible force from his hands as a RKA. Since he was a comic buff as a kid and he could fly he called himself Stellar Knight.
Here's the wierd part. He could control the energy but he was not immune to it. Everytime he used his RKA he would take a random amount body damage. He didn't have regeneration or any type of healing so he always had to find other ways to try to stop the villans. So many times he had to risk using his Stellar Beams at risk of death. Thinking about that character now I can hardly understand why he was so fun to play but he was. I think it had something to do with having to try to find other ways to stop the villans but also having access to a very destructive attack that my character was very afraid to use... heck, I was afraid! hehe