The beginning battle scene, both for the characters involved and the animators.
When a building collapses because of an earthquake, Maou manages to place magical barriers around every single person in the restaurant at once. Considering magic power for demons comes from fear, the logical explanation is that he used the fear the people in the restaurant felt to protect them.
Lucifer/Olba vs Alsiel/Emi. The explosions literally cover the entire sky. Lucifer BeamSpamming, Emi calls her holy weapon, Olba shoots off thunder bolts, and Alsiel hurls cars like baseballs.
You'll notice that Maou was not in the above line-up. This is because after he finishes stopping time, gently putting down the entire highway bridge and protecting the civilians, he ends the fight almost instantly. He crushes Olba with a glare (that somehow sends the priest through the wall), and FlashSteps right in front of Lucifer's face. Maou then creates a chain of magic circles miles long to deliver a Mega Ton Punch that could be seen from space.
Lucifer's face when Maou teleports into his face is priceless. He practically wets himself.
Chiho is not afraid to unleash the F***ing Fury on Maou if he should bring up any talk about whether she wants her memory of him erased for her own good... or if he doesn't take handmade double decker lunchboxes from other girls seriously. Demon lord and ability to level city blocks by will be damned, she will launch across the table, bodily drag you to her end and shake you silly till you answer her properly.
Let's not ignore the extent of Maou's, Alciel's and Emi's achievements concerning their dedicated and sincere efforts in the mundane world and their work life.