Since Iron Man 2, Black Widow has been one of the Marvel Cinematic Universe's deadliest heroes. She is a superhero and a member of The Avengers, but she is also a superspy and an assassin and never shied away from killing someone when needed. This makes her as much an antihero as anything.
In the comics, it is more of the same. She started out as a villain, the notorious Black Widow, a Soviet spy who mastered manipulating men to get into position to assassinate them. Even when she became a hero, she had moments where she ended up battling other heroes, and there were several times she beat them soundly.
Yelena Belova
Black Widow didn't just defeat Yelena Belova in the comics; she broke her down and then killed her. Yelena was part of the Red Room after Natasha, and many considered her one of the best trainees there, rivaling even Romanoff. She also tried to turn on her Russian leaders.
However, Black Widow captured Yelena and switched faces with her and Yelena didn't know who she really was. Natasha claimed this was a hard life lesson, but Belova felt violated. Natasha ended up using clones of Yelena to set her up and then killed her, off-panel, in Tales of Suspense #103 by Matthew Rosenberg and Travel Foreman.
Spider-Man
Early in Black Widow's career, she was a former enemy spy turned hero. However, she also made things tough for heroes who tried to face her throughout the years. One of these was Spider-Man and in Amazing Spider-Man #86 by Stan Lee and John Romita Jr., the two fought and Black Widow won.
Black Widow searched out Spider-Man to see who was better, and also because she hoped to combine their powers to make her stronger. She took him down, but then she gave up on the fight. While Spider-Man was losing his powers at the time, it was still a big win for Black Widow.
Elektra
Black Widow was a former superspy and assassin from the Soviet Union. Elektra was a former assassin for the villainous Hand. Both ended up striking out on their own and becoming antiheroes. However, they also remained rivals and have fought more than once. In 2010's Black Widow #3 by Marjorie Liu and Daniel Acuna, the two had one of their biggest battles. Elektra felt Black Widow was spying on her, and she took the fight to the Avenger. The two battled, and while they reached a stalemate, Black Widow had Elektra on the ropes and proved she was more than able to beat the former assassin.
Punisher
In Avengers Confidential: Black Widow & Punisher, the story starts with Punisher accidentally interfering with a S.H.I.E.L.D. operation. Punisher attacks a black market weapons dealer named Cain, but the problem is that Black Widow was trying to uncover a large terror plot and is also present. Because of this, Black Widow attacked Punisher, and the two fought. While the victory was not decisive, she helped stop Punisher and allowed S.H.I.E.L.D. to arrest him and force him to help with their investigation.
Wonder Man
Wonder Man was always one of The Avengers' strongest heroes. He started as a villain, working with Baron Zemo, and at that moment he beat Thor in a fistfight. He even took down Vision and Iron Man, showing his power. Yet, despite this, Black Widow still beat him when they fought.
This happened in Contest of Champions II, which pitted the heroes of Earth against each other in fights. Some fights never showed how the winners took the victory, and this includes the clash between Black Widow and Wonder Man, as she somehow choked a man who was made of energy out for the win.
X-Force
What seemed even crazier than Black Widow beating Wonder Man in Contest of Champions II?How about when she battled the entire team of X-Force? As an explanation, the X-Force was the former New Mutants, which Cable took over leadership of and changed their name to make them more threatening. Much like Wonder Man, this comic arc never showed how she beat them, only that Black Widow did beat the entire team, with them all lying unconscious before Thor showed up and then easily beat her.
Monarch
Monarch is James Braddock Jr., and if that name sounds familiar, it is because he is the brother of Brian Braddock (Captain Britain) and Betsy Braddock (the former Psylocke). Monarch is now the ruler of Avalon, an ally of Krakoa, and one of mutant-kind's most powerful leaders.
In the Infinity Countdown storyline, Monarch tried to get the Space Stone from Black Widow. However, with the Infinity Stone, Black Widow was too powerful and teleported Monarch into a wall, where he bled out and died. It was later that The Five resurrected him and placed him in charge of Avalon.
Hawkeye
Black Widow's story in the Ultimate Comics was like that of the 616-world, with one major change. In both cases, she was a Soviet spy and assassin who would romance and marry men, only for them to die in unforeseen circumstances. In both cases, she also defected to the United States and joined S.H.I.E.L.D. and their version of The Avengers, known as The Ultimates.
However, this Black Widow was a traitor sent in to infiltrate America. She revealed Hulk's identity to the world, framed Thor and Captain America for crimes against the country, and then she destroyed Hawkeye, taking him out and killing his wife and three children. In the end, Iron Man beat Black Widow, and while she was hospitalized, he showed up and killed her.