I know there are already a lot of X-Men mentioned here, but I would be Mystique.
She can morph into any physical form as well as being a cunning strategist in terrorist and commando operations, and adept at martial arts and information technology. She has a talent for finding, stealing, and understanding cutting edge weaponry. She is a talented actress and a polyglot, being fluent in over fourteen languages. Her mind is naturally unreadable owing to changing grey matter and wears devices to prevent telepathic intrusion. Furthermore, with over a century’s experience in posing as other people she has picked up the uncanny skill of being able to identify people posing as others based on body language and changes in behavioral cues.
So yeah, she’s badass! And she’s blue, who wouldn’t want to be blue?