Everytime George Bailey delivers a Shut Up, Hannibal! to Mr. Potter (including the very first time, when he was still a kid). Mr. Potter always starts out being all smug and trying to break George (and some others) with his words, deconstructing George's motives for fighting him and losing every time.
As horrifying as it ended up being, Clarence's plan to make it as if George was never born is pretty cool. It starts out as a moment where it looks like it'll be nigh impossible to complete his task and earn his wings, and then he manages to pull it off by using George's own words against him.
Harry Bailey really earned that Congressional Medal of Honor, saving a ship from kamikaze pilots.