In the MCU, many characters have had new life breathed into them with talented actors, solid scripts, and storylines that allow them to show what they are made of and save the day. Iron Man, for example, has risen from being a so-so hero to the star player of Marvel, and Captain America has never looked better.
What about Natasha Romanoff, otherwise known as Black Widow? She doesn't have a space hammer or a giant green form, but her elite training and gadgets allow her to handle any human opponent, and she can infiltrate any facility or assassinate any target. But she'll need a partner. Who in the MCU would be a good partner for Black Widow, and who would be a poor fit?
10 Team Up With: Gamora
Gamora is a humanoid alien who was adopted by Thanos in her girlhood, and she has undergone some serious training. She is a master of the sword, and martial arts come just as easily to her. Up close, not many opponents can stand up close to her for long.
What is more, Gamora is serious, practical, and mission-oriented, and that puts her in the same league as Black Widow. In a way, Gamora is the Black Widow of the Guardians of the Galaxy, so she'd be a natural fit for the human Black Widow.
9 Reject: Doctor Strange
This caped gentleman is the master of the mystic arts, and he even had the Time Stone on his person. Dr. Stephen Strange was a surgeon who learned to master magic, and he has been slinging sorcery and speaking metaphysically ever since.
Dr. Strange is a powerful ally on a chaotic battlefield, but his mind-bending sorcery would disorient Black Widow. Furthermore, Doctor Strange has no interest in Black Widow's kind of mission anyway. He'd make for a pretty awkward partner, to say the least.
8 Team Up With: Star-Lord
Although Star-Lord/Peter Quill is a lot goofier than Gamora, he is a loyal part of the Guardians. His skills are also roughly in a similar league as Black Widow and Gamora's. He is experienced at piloting ships, using pistols, maneuvering around cluttered battlefields, theft, and more.
Black Widow can identify with that, and if she needs to complete a mission in outer space, Star-Lord is as solid an option as Gamora. Besides, Star-Lord's personal warmth and jokes might cheer Black Widow up a bit and help her relax. Being wound up too tight can have consequences.
7 Reject: Thor
Just because a character has a lot of strength and abilities, doesn't mean they are the ideal choice for a given mission. Some assignments call for proper warriors, while other jobs require hackers, sleuths, infiltrators, or assassins. Thor is strong, but he can't do everything. And sometimes he acts rashly or gets the wrong idea.
There is nothing subtle about Thor since he's all about lightning and hammers. He struggles to stay incognito during missions, and he is easily lured into battle or angered. Black Widow needs a stealthier, more cool-headed partner than this.
6 Team Up With: Ant-Man
Scott Lang became Ant-Man when Hank Pym tested his burglary skills and allowed him to steal the high-tech Ant-Man suit. Now, this ex-con is Ant-Man inside and out, and he can adjust his size with his slick suit. And, of course, he can command and ride on ants.
Those abilities, not to mention Ant-Man's existing expertise and experience with infiltration and thievery, make him a picture-perfect partner for Black Widow for a mission. Black Widow has superior melee skills, but not even she can escape a room through the door's keyhole.
5 Reject: Killmonger
Erik Killmonger is Black Panther's challenger to the Wakandan throne, and that is his main preoccupation. He is also interested in spreading Wakandan power throughout the world and using it to help people in need.
That might sound good, but Killmonger is brutal, bloodthirsty, and quite self-absorbed. He even tried to make it so no one could become a Black Panther after him. Overall, given his preoccupation with Wakanda and emphasis on up-front fighting, he would make a poor fit for Black Widow. Stealth and spy stuff are not his forte.
4 Team Up With: Spider-Man
If Black Widow can have an ant on her team, she can have a spider by her side, too. She wouldn't like Peter Parker's naive side, nor would she like how flustered this youth can get; however, Spider-Man is working hard to better himself as a hero.
Aside from that, Spider-Man can offer a lot to Black Widow, such as using his webs to silently snare enemies or to block enemy reinforcements from arriving. He can also climb up walls, making him even more maneuverable than Black Widow in some ways.
3 Reject: War Machine
James Rhodey is War Machine, an alternate Iron Man who makes good use of all kinds of powerful guns on his suit. He can fly around and perform melee attacks as well, and he is serious and disciplined in battle.
However, War Machine is rather loud during a mission with all those firearms, and he's not a tech genius or as resourceful as Tony Stark. Black Widow can't ask him to hack into an enemy's mainframe or decipher ultra-advanced science, for example. She'd need someone else to handle it.
2 Team Up With: Captain America
Black Widow has already gotten a taste for Captain America as a mission partner. They made a fine, if slightly awkward, team during Captain America: The Winter Soldier, and they complimented each other fairly well.
Captain America knows what it's like to fight as a commando with no superpowers or magic, meaning he can fight on Black Widow's terms. And if the enemy does bring heavy firepower into the equation, Captain America can use his shield to keep himself and Black Widow safe.
1 Reject: Heimdall
Heimdall is an Asgardian like Thor, and his mission is very clear. It's up to him to guard the Bifrost and ensure that only authorized parties enter or exit the realm. He is also able to see nearly anything in the nine realms with those powerful eyes of his.
Heimdall is good at what he does, including fighting on the field of battle, but he's not going to help Black Widow. He utterly lacks her particular skills and expertise, and he is sworn to guard the Bifrost and act only in Asgard's best interests. Black Widow couldn't convince him to abandon his post to help her, and she'd have little reason to try anyway.