3. Black Widow
The very sexy, sultry and petite Natasha Romanoff - also known as the Black Widow - doesn't look like the kind of person you should be afraid of. However, to underestimate her based on her appearance would be a huge mistake. Black Widow is a skilled espionage agent who is both an expert interrogator and a supremely talented fighter. Her crowning moments in her appearances so far include her allowing herself to be "captured" in order to get herself in to a position where she can extract information from her target - when she is called to report for duty by S.H.I.E.L.D's Phil Coulson, whilst still tied to a chair, she begins kicking some serious butt. Within 30 seconds, she's free of the chair and all three captors are subdued. She picks up her heels and calmly walks off. She was also key to the Avengers ending the alien invasion of Earth in New York City. In order to shut down the portal through which the Chitauri were emerging, she needed to get to the top of Stark Tower. She saw the Chitauri fliers zipping past overhead and, using a bunk up from Captain America, she grabbed one mid-flight in order to kill the alien pilot and commandeer it to fly herself up. She was also kicking some serious Chitauri rear end in hand to hand combat - even though they were considerably larger than even the largest of human males. The fact that she also wears a tight-fitting, all-over leather body-suit doesn't do her any harm either.