13. Black Widow
It doesn't get more badass than Natasha Romanoff. The Black Widow might not have any super-powers, but she's one of the deadliest characters to exist in all of comic books or movies, with a multitude of skills to her name, and the ability to neutralize threats in a split second.
Originally an assassin trained in the Red Room, she joined SHIELD and devoted her life to making amends for her previous wrongdoings, while also helping the Avengers save the world. Her commitment and effectiveness in that mission made her an Avenger. That right there shows you how strong she is.
Like Hawkeye, Black Widow doesn't have abilities like the other-worldly beings on the Avengers, but she does have a specific skillset that makes her incredibly efficient at what she does, and a determination to push herself as absolutely far as she can go to complete the mission. That, along with her will to clear out the red in her ledger, makes her perhaps the strongest mortal on The Avengers. She achieved her goal in all of that, playing a pivotal role in saving the universe from Thanos, when she gave up her life for the Soul Stone. She made the ultimate sacrifice so that her friends (and her best friend Clint) could live; there is no greater strength than that.