Normally when I include a comic book character on this list, I've used a picture from the books, not the movies or shows that followed. That's because in nearly every case, I first got to know and appreciate that character through the print medium, often as a younger reader. Black Widow is an exception to that rule for me. In my years of reading Marvel comics I was always frankly a bit underwhelmed by Natasha Romanoff. She showed up initially in the pages of the Avengers as a Russian spy, running around with low-level criminal/wannabe hero Hawkeye. She follows the rambling path to redemption, eventually graduating from 1960's fishnet Bullwinkle caricature to the gray and then black suit of the 1980's and 90's and membership among Earth's Mightiest Heroes. She's been around forever, linked romantically to Hawkeye and Iron Man and Daredevil and a bunch of others, but always seemed vaguely inaccessible, withdrawn, almost a weary adult in a world of technicolor-costumed teenagers. Layered backstory, sure, red in her ledger, but it never really drew me in.
It was her portrayal by Scarlett Johansson in the Iron Man, Captain America, and Avengers movies that gave me a fresh appreciation of Black Widow. That'll probably get me in trouble with the film-as-flawed-vehicle-for-social-justice crowd, as it has become a cottage industry to critique the movies for imagined or even legitimate failures to perfectly capture and depict the bewildering complexity of modern womanhood as it intersects with superheroics and cinema. Lots of people have undisputed right answers to those questions, and God bless them - I know I haven't a clue. I don't know if Widow's quiet scenes with Hulk/Banner deepen her character or infantilize it. I don't know if the Red Room flashbacks advance her narrative or burden her with tropes. Ask ten people and you'll get ten answers.
What I do know is that Johansson (and the films' creators) have done excellent work in giving Black Widow a texture and nuance that elude so many contemporary efforts to produce female heroes. She's strong and badass, sure, but she also has doubts and makes mistakes. She holds her own with the fraternity of the Avengers without compromising her femininity or ferocity. It's interesting to me that when she exercises leadership she's criticized as "Mommy Widow" and when she's working as a part of the team she's criticized as taking a back seat to men. She can't win, so she doesn't even play the game - she simply pursues her own agenda. She's been at this for a long time, far longer than her youthful appearance would suggest. And she doesn't need your approval.