10. The Winter Soldier
Bucky Barnes couldn't have imagined the life that we would endure after his apparent death in the 1940s. But unfortunately for him, he was kidnapped by Hydra, brainwashed, and modified into a super soldier assassin known as the Winter Soldier. Decades later, he would cross paths with best friend Steve Rogers all over again and slowly, eventually, remember who he was.
Bucky is a super soldier, so he has all of the strength and durability of Captain America. However, the thing that sets him apart from Cap is his metal arm, which is made out of Vibranium - and thus, it's safe to say that he packs a punch. All you have to do is watch the final scene in Captain America: The Winter Soldier in which he outpowers and pummels Steve with that arm.
He is also an expert marksman, a professional assassin, and incredibly skilled in hand-to-hand combat. Granted, now that he's Bucky again, he doesn't use all of those skills anymore, but he's still capable of demolishing walls with his bare hands and going toe-to-toe with almost any of the other Avengers if he has to.
While he might not consider himself an official Avenger, he's an excellent ally for the team whenever they go up against larger-than-life threats.