After the events of Dark Reign, Norman Osborn's true intentions were revealed and the President of the United States knew that something needed to be done to reestablish law and order in the country concerning superheroes. He named Steve Rogers, who was no longer operating as Captain America, as the top law enforcement officer.
After Rogers convinced the President to abolish the Superhero Registration Act, he then went to work on forming a new superteam that worked in conjunction with The Avengers. This team was the Secret Avengers and was a black ops team that worked off the grid and dealt with cleaning up known threats. Over the years, the rosters changed but included some very powerful characters.
10 COMMANDER STEVE ROGERS
When it comes to powerful comic book characters, strength and superpowers are not all that makes someone powerful. In the case of the founder of the Secret Avengers, there might not be a more powerful hero that Steve Rogers, the former Captain America.
Of course, that does not mean Steve Rogers is powerless, as he has the supersoldier serum flowing in him and that super strength and durability are only made stronger by his integrity and desire to do the right thing. There is not a better man to lead the Secret Avengers than Rogers, although Hawkeye gave it the old college try.
9 MOON KNIGHT
Much like Steve Rogers, Moon Knight has no real superhuman powers. However, also like Rogers, he does have augmented strength and durability. Instead of a super-soldier serum drug giving him his powers, though, Moon Knight might or might not have his power thanks to the Moon God Khonshu.
Moon Knight seemed like the perfect fit for the Secret Avengers. While he is not a team player, he is the expert black ops soldier, with years of mercenary work in his past before he became a superhero. While Rogers might be more powerful than Moon Knight, the strength and powers of this hero rise and fall and when he is powered up on the night of a full moon, he is almost unbeatable.
8 GIANT MAN
The Giant Man that was a member of the Secret Avengers was the original -- Hank Pym. Already one of the smartest men to ever live, Pym enjoyed a resurgence following the Secret Invasion storyline when it turned out he was one of the heroes replaced by Skrulls. He took a leadership role, both of The Avengers and Avengers Academy.
It was after this that he joined the Secret Avengers and Pym was a perfect fit for black ops work based on the fact that he was also a member of the Illuminati. On top of his genius-level intellect, he has use of the Pym Particles, which allows him to grow and shrink and possess increased strength and stamina.
7 WAR MACHINE
Imagine Iron Man with some powerful guns working for the U.S. Government, and you have War Machine. At one time, James Rhodes was Tony Stark's right-hand man and then became Iron Man for a time, when Stark fell to alcoholism, and Rhodes joined the West Coast Avengers.
When Stark returned as Iron Man, Rhodes became War Machine and implemented a targeting system and various weapons for battle. War Machine was part of the first Secret Avengers team while also serving as the U.S. military's version of Iron Man. At this time, Rhodes used the Variable Threat Response Battle Suit.
6 AGENT VENOM
While Eddie Brock's Venom is the one most fans recognize and love, there is a lot to like about Flash Thompson's Agent Venom as well. Flash Thompson had a fantastic character arc in the comic books, going from a high school bully who tormented Peter Parker to a war hero who ended up paralyzed.
When Venom bonded with Flash, the new Venom was a military-based hero, with Flash working as a soldier using the symbiote to allow him to not only walk again but fight as a real superhero. Venom was already powerful as it was, but with the military training and weapons used by Flash, this version was a step above the rest.
5 CAPTAIN BRITAIN
Captain Britain is Brian Braddock and was Britain's version of Captain America. While his sister is more powerful (Psylocke), Captain Britain is pretty powerful in his own right. He received his powers from the legendary Merlyn through the powers of the Amulet of Right.
As a member of the multiverse Captain Britain Corps, he has superhuman strength, stamina, durability, speed, senses, reflexes, and the ability to fly at supersonic speeds. The one downfall is that, after a recent retcon, his powers are tied to his emotional state, so when he is confident, he is extremely powerful. He was a member of the second Secret Avengers team.
4 HUMAN TORCH (JIM HAMMOND)
This hero is not the Fantastic Four member, Johnny Storm. Instead, this is the original android Human Torch that fought alongside Captain America during World War II as part of The Invaders. He was brought back when the Mad Thinker stole the corpse of the Torch and took control of the android.
After Jim Hammond was able to defeat Mad Thinker, he was later offered a spot in the Secret Avengers when Hawkeye took over as team leader. He ended up brainwashed by a group called the Descendants and finally quit the Secret Avengers in disgrace. He is immensely powerful and once walked out of a nuclear blast stronger than before.
This Nova was the Richard Rider version -- the original Nova from Earth. Richard Rider became Nova as a teenager in a similar origin story to Green Lantern, but he worked as a teen hero who had little knowledge of how to use his powers. However, as he grew up, he learned of the Nova Corps and became one of their most powerful warriors.
He has superhuman strength, durability, and speed, and he can absorb energy and release it in pulses. Richard joined the Secret Avengers from the start, recruited into the team by Steve Rogers, as a team member who could investigate problems on other planets.
Valkyrie was introduced in the MCU in a very different form than from the comic books. Instead of a former Valkyrie warrior who went into hiding after a slaughter, Brunnhilde was a proud warrior selected by Odin to lead the Valkyrior who was enemies with The Enchantress and ended up on Earth with a secret identity, similar to Thor, but with no knowledge of the being.
Over time, Valkyrie gained her freedom and became one of the most powerful Asgardians on Earth, becoming a long-standing member of The Defenders. She later joined the Secret Avengers, although she left by the time Fear Itself began. In the comics, she is the strongest of all Valkyrior.
Hulk claims to be the strongest one there is, and few deny this. His power has no limits, as the angrier he gets, the stronger he gets. A Hulk blinded by complete rage is unbeatable. Hulk is also pretty much immortal and will always return to life even if he somehow manages to perish.
Hulk was a member of the very first incarnation of The Avengers, although he quickly left when he learned that no one trusted him. He was part of the second incarnation of the Secret Avengers when they needed someone strong enough to destroy the Iron Patriot drones created by A.I.M.