I'm going strictly with the MCU, so I'd probably have to say Pepper.

I think Natasha would put people into categories: those who could be a threat, those who are part of the mission, etc. She wouldn't waste her time with people outside her job, because she could never be honest with them about what she does and she'd see them as a distraction. Being close to her colleagues - other SHIELD agents - is a risk, too, because sometimes getting the job done means losing people in the field, and she would want to put the mission first, not f*%@ it all up because she got sidetracked trying to save someone's life.

So Pepper is the only one who is really outside of those categories, and she's a strong, smart, sophisticated woman, which makes her someone Natasha can both like and respect.