Before issue 5, the Flash was a novice with his Super Speed and couldn't even brake properly. Hawkgirl tries to keep him out of the battle with Grundy and helping civilians instead. When the Atom drops in and takes out Green Lantern and Hawkgirl, only Flash is left and he decides it's time to finally step up. He quickly climbs the giant Atom and pummels him until he topples over. Even Hawkgirl says he did good. Keyword being good, as the Atom gets back up.
Alan breaking free of the Grey and using his powers to first deflect the missiles headed for Washington while on his way to dump Grundy on the moon.
A villainous one: In issue 16, Brutaal of the Hunger Dogs usurps Steppenwolf as the Big Bad when he karate chops Steppenwolf down the middle. Needless to say, nobody saw that coming.
Pa Kent deserves mention for standing up to Superman. He may have died but that was an amazingly brave thing to do.
Alan Scott using his ring to keep the Earth from being pulled into a Giant Boom Tube. Willpower indeed.