Phoenix

I first became acquainted with this creature as a child while watching "The X-Men. "
The X-Men were off doing something, when all of a sudden, this woman with flaming red hair shows up and becomes this HUGE mythical bird of fire.

My thoughts went something like this:
"It's a girl! And she's the most powerful being in the universe! WOW!"

Having never seen a superheroine before that was even REMOTELY this awesome, I was immediately impressed.

Needless to say I've been a fan of the Phoenix, X-Men and comics ever since.