Black Widow debuted in the Marvel Cinematic Universe with Iron Man 2, but fans got a better understanding of who Natasha Romanoff was with The Avengers, where audience learned about one her most valued friends, Clint Barton. While it's undeniable that Clint and Natasha are close, her role in Captain America's films prove Steve is her best friend.
How Black Widow's Friendships with Captain America and Hawkeye Started
Natasha has a longer history with Clint, as he was sent by S.H.I.E.L.D. to kill her initially; however, he changed his mind, giving Natasha a second chance. Given Natasha's past, someone seeing beyond the crimes she's committed meant a lot. Plus, Clint gave her a say in what she did next, and that freedom is something Black Widow agents often lack. She could've killed him, or she could've given him the chance he gave her.
She chose the latter, leading to a new life as a S.H.I.E.L.D. agent, where she worked alongside Clint, developing a friendship so strong that Clint would eventually invite Natasha into his family. Black Widow owes a lot to Hawkeye because of this past; however, in Captain America: The Winter Soldier, audiences see that Captain America is Black Widow's best friend.
At the start of the film, the two have a friendly partnership, with Natasha constantly trying to set Steve up. However, once the Winter Soldier enters the picture, things take a more serious turn. For instance, the two discuss how Cap struggles to find someone with shared life experience, so Natasha suggests he make something up. In response, Steve says it's hard to trust someone he doesn't know, and when Natasha asks who he wants her to be, he tells Natasha he just wants her to be a friend.
Natasha brushes this off, but his words resonate with her. Throughout her life, Natasha adapted to what was needed from her. While Clint gave Natasha a second chance and their friendship is genuine, there is the caveat that this relationship directly led to Black Widow trading in one life of espionage for another.
Meanwhile, Steve sees Natasha as a person first. She is a huge help when it comes to taking on Hydra, but he rather Natasha be her genuine self than a good spy. A friendship that's based on honesty may be hard for Black Widow, but Steve is willing to trust Natasha with no strings attached, and this trusting and unconditional bond is what makes Steve her best friend, despite their difference.
Captain America: Civil War Changed Everything
With Captain America: Civil War, Natasha's beliefs challenge both Clint's and Steve's, and she goes head to head with both heroes, but how she handles them differs. With Hawkeye, they tease one another while fighting, but with Captain America, she is reluctant to hurt him. That's not to say she doesn't care about Clint, but it appears her feelings for Captain America are stronger, as she not only lets him go, but she also abandons the Avengers all together to later join up with him and Sam Wilson.
It makes sense she'd join up with them, especially since she does not have a traditional family to return to, unlike Clint. Natasha and Steve's lives are about being heroes, while Clint's life is about his family. Hawkeye had a home to go back to, while Steve and Natasha had each other and their shared drive to save others, making them even closer.
Black Widow's Endgame
After years of being on the run, Natasha and Steve's friendship strengthened. Meanwhile, it appears she's not been in contact with Clint since Civil War, as their lives were on separate paths until Avengers: Endgame. In this film, Natasha has taken on a leadership position amongst the Avengers, with Steve standing by her side. Meanwhile, Clint is consumed by grief, cutting off ties with his loved ones who are still around. Natasha manages to pull him back, but it's only after she says there's a chance they can bring back his family.
Black Widow's time in the MCU, at the time of this writing, ends with her sacrificing herself for Clint, his family and everyone else who was snapped out of existence. Neither character wants the other to die, and they are willing to do anything to avenge the fallen, but it's clear Natasha was willing to sacrifice herself for anyone she loved if it meant bringing everyone else back. She loved Clint and Steve, but at that point in the MCU, Steve was Natasha's best friend, while Clint was her family.