For me? Skullfire, from X-Men 2099, before he became some jumble of electricity. The character design originally was awesome, and his backstory was great. He was a mutant who couldn't control his power and felt like he was too dangerous to be around. After running in to La Lunatica, who FUBAR'd his head, he gained control of his power and went a little more psycho/dark, and his character really took shape. I miss Skullfire. And I miss X-Men 2099.