Matter-Eater Lad takes a big bite out of the Emerald Eye of Ekron, finally defeating Emerald Empress (otherwise the mistress of Villain Exit Stage Left) permanently.
Bouncing Boy showing his leadership potential during "Chain of Command". This rightfully earned him his Darkhorse Victory at the end of the episode.
In his debut episode Timber Wolf calls out his father on his crappy treatment of him and others then proceeds to trash his lab in a fury (including, punching a hole through solid metal to rip out the wires and ripping out an operating table from the floor and throwing it at a bunch of computer screens). Earlier he roars at the animal test subjects who looked ready to maul his Dad. Given to how he was acting at this stage this was so richly deserved.
Dr. Londo: "Not even intergalactic law can touch me, and why would they? My work is a gift, I create life!"
Watching the Legion of Substitute Heroes get their act together and defeat Starfinger definitely counts. It just goes to show you that the underdogs can come out on top, even if no one believes in them. Their powers might not have been as cool or as strong as the rest of the Legionnaires', but when it came time to step it up, they made a pretty good team.