The Hulk isn't a hero to be trifled with. As a gamma ray-fueled beast, Bruce Banner's other half is incredibly strong and shows off an unmatchable rage. Although there have been multiple instances where his allies have had to turn on him to subdue him, it's interesting to theorize which of his fellow Marvel heroes would even be able to take Hulk down, mano-a-mano, and who could do it the most efficiently.
Gamma mutates are some of the strongest beings in the Marvel universe. The Hulk is especially formidable due to how his emotions drive and further pad his overall strength and abilities in battle. The more intense his emotions, the more anguish he feels, the stronger he becomes. His skin is nearly impenetrable, he can leap high and far and land without injury, and he can fend off some of Marvel's most powerful heroes. Even so, there are multiple Marvel characters that could theoretically take down the Hulk on their own. But the one who could do it with the most ease would have to be Scarlet Witch.
Wanda Maximoff's ability to bend the very constructs of reality itself and almost completely rewrite aspects of it on a whim make her one of the most dangerous heroes in the Marvel universe and possibly a force strong enough to take down even the Hulk. If she were to utilize the full strength of her powers, or if she were able to have better control over them, she could easily use them to prohibit an uncontrollable Hulk from running amok. She could just rewrite his accident. She could make it so his origin story never came to be and neutralize the threat in an instant. Or she could use her reality warping abilities to sap him of his strength, to take away the anger that fuels him. The possibilities are literally endless with the power she has at her disposal. Additionally, the Scarlet Witch has been trained in hand-to-hand combat by some of her fellow Avengers and is extremely knowledgeable in the art of sorcery.
This is probably also the reason that Marvel should never put the two into a one-on-one fight, because it would realistically be a very boring fight and over in record time. Yes, the Scarlet Witch tries to keep her powers at bay, but if she had to, she could unleash them and the Hulk wouldn't stand a chance. Of course, she isn't the only hero who has the potential to put the big green monster in his place. Thor should definitely get an honorable mention in this theoretical. Being a literal god, Thor has his fair share of power to throw around. Plus, the two have actually fought each other countless times in the comics and Thor should have a pretty good grasp on how to beat him by now. After the Scarlet Witch, he's probably the most capable to take down the Hulk, if need be.
Regardless of who could take him down, it's important to note that the Hulk is still one of the physically strongest heroes that exists in Marvel comics and going head-to-head with him is a move that is not to be taken lightly. He would not go down without a fight and it'd probably get a little heated, especially if it happens to be an evil, twisted Hulk that's taken the reins. But it is good to know that if he does need to be taken down, and quickly, they could always pull out their trump card and sic the Scarlet Witch on him. Although, considering how unruly her powers can be, she'd probably end up taking down a lot more than the Hulk.