The climax of the first short, where Supes punches through a laser beam, blocks a full-power blast with his hands, and ties up the turret, prompting a subsequent meltdown. That moment alone sold the series as a bona-fide hit.
Just the line before the climax sells it when the Mad Scientist simply says, "It's Superman!" The whole feeling in that line of a villain now facing a hero unlike any other before is powerful.
The third short, "The Billion Dollar Limited," is full of them. Lois Lane is on a train carrying a billion dollar gold shipment to a mint when its attacked by train robbers in an armored car. Lois's reaction? Pick up a tommy gun and start shooting them. Then when Superman shows up he gets a whole run of them: He moves the rails over so the train doesn't crash into explosives, he catches it when it goes over a bombed out bridge and when the locomotive is destroyed he drags the train uphill, through a huge cloud of tear gas and brings it to the station.
The second short 'The Mechanical Monsters' has Superman whupping over a dozen fire shooting-super strong robots. Then the bad guy threatens to drop Lois into a vat of molten metal only for Superman to catch her in the blink of an eye. Lastly the villain pours all that molten metal onto Superman who merely shields Lois with his cape and acts like it all a minor problem for him. He then easily catches the villain while taking Lois to safety.
In 'Volcano', Lois escapes a lava flow and swings hand-over-hand to a stopped cable car above the boiling lava to escape— and she actually makes it there before the cable catches on fire and falls and Superman has to save her. And then Superman gets one by saving the whole city by diverting the lava.
Pretty much each time Clark's higher pitch is replaced midsentence with Superman's deeper voice. The phrase 'this looks like a job for Superman' was pretty much a Pre Ass Kicking One Liner.
On a meta-level, Paramount itself had one by commissioning the shorts. Dave Fleischer initially turned down the offer to make the cartoons. When Paramount called up to ask why, Dave said Superman was too expensive to do right on film and make a profit. Paramount footed the bill anyway and some of the greatest pieces of animation ever were created. With so many stories of Executive Meddling in Hollywood, it's truly special to see a case of studio execsDoing It for the Art.