My personal favourite is Dr Jean Grey from the X-Men, who transforms into the Phoenix and, being a level 5 mutant and more powerful than any other, creates havoc before sacrificing herself to save everyone else. She returns (like the phoenix) and learns to control her amazing powers of telepathy and telekenisis. She's also part of a love triangle with two fellow mutants Cyclops and Wolverine fighting for her affections. What's not to love?!