Best: She has actual dimension
Superhero morality can often feel as complex as a schoolyard fight. Villains want to end the world (or worlds) for incomprehensible reasons; heroes are good in an uncomplicated way, even if their humor is sharper and their means a bit more dangerous. Time has helped the Marvel heroes accrue life lessons and backstory, but they aren't Chekhov characters.
Black Widow's strongest traits -- layered persona and quicksilver nature -- were present in the comics, and thankfully made it into the films. Since her first appearance in Iron Man 2, Black Widow has been complex in a way that makes the other characters around her seem like cardboard cutouts. Her backstory -- she betrayed the only home and world she ever knew as a Russian/KGB spy -- lends her some humanity, and makes her decisions take on a different edge.
She's a soldier, but she's also been a double agent. She's brutal, but vulnerable. She often uses men underestimating her to her advantage. She's loyal to her friends, but understands that empires may fall. She's a woman who operates best in the shadows, yet she releases S.H.I.E.L.D.'s files and makes the choice to lose her own secret identity at the end of Captain America: Winter Soldier because it's the right thing to do. She exists on ever-shifting ground, and the role of Black Widow isn't one she took on voluntarily but rather one she keeps to help the world, to clear the "red in her ledger." Whenever Black Widow (and Black Panther, for that matter) are the focus in Civil War, the plot is about much more than just brutes duking it out. Their storylines feel like they're from different, more interesting films.