In terms of rule of cool, Cap's reveal in Infinity War. Much Roundtop said, the world can change but Cap stays true. One of his people is in trouble, where else would Cap be?
In terms of most powerful scene, it's got to be the scene in Stuttgart with Loki and the old man. It had more gravitas than Cap's signature "I can do this all day." Even before he stands up, the old man stands out by facing a different direction from the rest of the crowd. In two dismissive, almost contemptuous sentences, he reminds the crowd that despite the costume and the power, Loki is nothing more than a thug with a gun.
There have been many moments in the MCU where characters choose a course that will likely kill them. Thor holding open the forge in Infinity War, Stark steering the missile in Avengers.
There are Obi-Wan moments, like when Rogers rides the Skull's bomber into the sea or when Heimdall saves Hulk as his final act.
But this old man knows that if he stands up, he dies. His death won't change anything. It shouldn't be a surprise that another of Wheadon's works offers insight into this simple act. "If nothing we do matters, then all that matters is what we do."
