Hydra is perhaps Marvel Comics' most villainous organization, and, with the help of a Cosmic Cube, it was able to infiltrate S.H.I.E.L.D., the United States government, and even Captain America himself. This culminated in the Secret Empire event of 2017, where Steve Rogers took the helm as Hydra's Supreme Commander, usurped the United States government, and coaxed a number of Avengers to join him.
The amount of Avengers to actually side with Captain America on this was startling, to say the least, and many of them were powerhouses of the team. It's no wonder that it took a coalition of virtually every other Marvel hero to win the day in the end. Startlingly, this isn't even the only time that heroes have joined up with Hydra.
10 Spider-Woman
Our first Hydra-turned hero actually has nothing to do with the aforementioned Secret Empire. Jessica Drew was inducted into Hydra early in her life. After receiving her abilities from her father and the High Evolutionary at Mount Wundagore, Jessica left Wundagore late into her teenage years and was found by Hydra. They offered her sanctuary, physical training, and a lie about fighting against the evil corporations of the world.
They also used myriad brainwashing techniques to keep her committed, and it wasn't until she almost assassinated Nick Fury that she began to question and, eventually, leave Hydra. When she joined the New Avengers, she briefly established contact with Hydra to find a solution to her then-unstable powers. Nick Fury helped her out of this bind too--though it also led to Jessica being replaced by Skrull Queen Veranke.
9 Xorn
The enigmatic Xorn was a member of the X-Men at various points and established a relationship with Magneto and Emma Frost.
It was this that led him to be established as the leader of the New Tian, the mutant state that was in a temporary alliance with Hydra during Secret Empire. While star-powered Xorn was the public ruler of New Tian, he wasn't truly in control of the country.
8 Emma Frost
Emma Frost was the true ruler of New Tian, though even she was doing so under the orders of Steve Rogers. Frost, a powerful telepath, was controlling Xorn for Rogers.
While Frost has a long history of self-centered power-brokering, she was assumedly doing to maintain a sense of peace between New Tian and Hydra. It didn't work, and, when it was time, she led New Tian to fight Hydra.
7 Wolverine
This one is actually unrelated to Secret Empire as well. During the "Enemy of the State" story in the Marvel Knights Wolverine title, Logan was brainwashed into working for Hydra by the Gorgon. At this time, Hydra, A.I.M., and the Hand were working in conjunction to seize power in the United States.
During this time, Wolverine goes to war with S.H.I.E.L.D., Elektra, Daredevil, the Fantastic Four, and even his friends in the X-Men. Wolverine killed Northstar and helped get Elektra captured and reprogrammed by the Hand before S.H.I.E.L.D. and Captain America finally subdued Wolverine and freed him from the Hydra brainwashing.
6 The Vision
The Vision was among the heroes brainwashed by Hydra during Secret Empire. Arnim Zola created an AI virus with which to infect Vision, and it was injected into the hero when the Avengers initially responded to Hydra's invasion.
He was left unable to control himself and allied himself with Captain America and Hydra.
5 Magneto
Magneto, on the other hand, worked with Hydra because he thought it best for mutant kind. Before Captain America initiated the Hydra invasion of the United States, he contacted Magneto and promised him a mutant nation on the West Coast after Hydra takes over. Erik, then leading his own team of X-Men, accepts this deal, and this led to the formation of New Tian.
Meanwhile, Magneto himself went underground. It is more than a little disconcerting that Magneto was willing to cooperate with Nazis while being a Holocaust survivor himself, but he led the charge on Hydra when they betrayed New Tian. In one of the most memorable moments of Secret Empire, Magneto helped bring down the Hydra base in D.C.
4 Thor Odinson
Thor Odinson, then still unable to wield Mjolnir, was another member of Hydra's Avengers team, and he doesn't have the excuse of being brainwashed that the Vision had. The Odinson, on the other hand, was promised that Captain America would free Jane Foster, who held the title of Thor at the time, from a parallel dimension.
The Odinson's innate trust of Steve Rogers certainly played a role in this as well, but one would like to think that the Odinson would recognize tyranny when he sees it.
3 The Hulk
The Incredible Hulk was technically dead at the time of Secret Empire, having been killed by Hawkeye and a gamma-powered arrow. However, Hydra found a way to temporarily resurrect the Jade Giant and unleash him on the Underground resistance holding out near Las Vegas.
They dropped the Hulk on the secret base like a bomb, and it ended with both the base and Las Vegas being razed to the ground.
2 Scarlet Witch
The reality-manipulating Scarlet Witch would have been the greatest threat to Hydra's plan to take over the U.S., and that is why she was among the first to be pacified when Hydra invaded.
Steve Rogers took control over Wanda by having the chaos god Chthon possess her. This put the Scarlet Witch out of commission, and Chthon joined up with Hydra's Avengers team.
1 Captain America
In the end, however, Captain America himself proved to be Hydra's most valuable asset during Secret Empire, and he pushed the organization closer to global domination than the Red Skull, Baron Zemo, or Baron Strucker ever have. He helped them take over the United States of America, got a number of Avengers to join their ranks, and he helped convince much of the nation that all of this was a good thing.
In terms of raw power, Steve Rogers also wielded a Cosmic Cube for much of this time. While this Steve Rogers was technically an evil doppelganger at the time, he was, for all intents and purposes, Captain America, and he helped Hydra nearly take over the world.