Worst: She hasn't really had much of an arc of her own
Black Widow can be whoever the MCU's writers and directors need her to be in a given film, since they haven't given her a clear arc, which is infuriating for such a fascinating character. In Avengers: Age of Ultron, we see a dream sequence of sorts that touches on her backstory in the Red Room, where she was trained to become Black Widow.
Ultron takes many of its cues for Black Widow from the Richard K. Morgan miniseries "Homecoming," especially when introducing the idea of forced sterilization as essential to assuming the Black Widow identity (which is an incredibly weighty idea that isn't done justice at all). Of course, Ultron doesn't handle her all that well, thanks to the shaky romance she has with Bruce Banner.
Civil War doesn't treat her much better. Her material does have weight: Natasha spends most of the movie torn between Captain America and Iron Man, and undergoes a change of heart in the heat of the airport battle. But after letting Cap off the hook to go chase the villainous Zemo, she disappears. The third-billed actress, the lightning rod for Civil War's questions on loyalty and superheroic service, steps aside so the title heroes can throw down -- a total missed opportunity. Where was Black Widow in that final fight?