13. Quicksilver
One of the Avengers' biggest unsung heroes, Quicksilver briefly joined the team in 2015's Avengers: Age of Ultron to help them take down the titular unstoppable robot. His real name is Pietro Maximoff and he had initially joined Ultron's battle against the heroes along with his twin sister Wanda, but the pair sided with the Avengers when his single goal of wiping out all of humanity was revealed.
He wasn't a trained fighter in the way that Black Widow, Hawkeye, or Steve Rogers were, but he had an ability that gave him a ridiculous advantage over all of them: Super-speed. Yes, Pietro was a speedster, capable of travelling at - or beyond - the speed of light due to experiments that were performed on him by Baron von Strucker. That also meant that you couldn't see him coming, so it was almost impossible to prepare for his attack - because he was capable of reacting before you had even put your thoughts into motion.
Pietro gave the Avengers a pretty rough time when he was on the opposite side of them, but as an ally, he was imperative in saving as many lives as possible in Ultron's war against Sokovia. He even saved Hawkeye's life when he moved him and an innocent civilian out of harm's way, taking Ultron's gunfire and sacrificing his own life in the process.
Quicksilver is widely regarded as one of the MCU's biggest missed opportunities, because as important as his death was to Wanda's arc, it robbed the franchise of seeing him grow into his full potential. And that could have really been something.