Some good ones already! Now to add some not already mentioned…
Smallville Season 4 episode Crusade: Clark takes flight for the first time (being reprogrammed as Kal-El)
Smallville Season 4 episode Transference: The prison riot scene where Clark and Lionel face each other in each other’s bodies
Smallville Season 4 episode Commencement: The stones come together to form the fortress of Solitude
Smallville Season 6 episode Justice: Clark, Ollie, Arthur, Bart and Vic cool-guy walk away from an explosion and give a first glimpse at the Justice League
Smallville Series Finale: “Always hold on to Smallville” (or something like that)
Justice League Unlimited: Destroyer: Superman vs Darkseid speech “I live in a world made of cardboard, always afraid to hurt someone, you can take it can’t you big guy…now’s the time for me to show you just how powerful I really am”
Justice League Unlimited: Question Authority (i think): When the Question finally confronts Lex Luthor “Do you know how much power I would I have to give up to be president?!”
Young Justice Season 2 “Depths”: When everything is revealed at the end “It only gets harder from here”
Arrow Season 1 “Sacrifice”: Besides just the entire episode itself, the moment Ollie kills Fyers/his first kill
Arrow Season 2 “Three Ghosts”: Tommy’s spoiler appearing to Oliver “you are a hero…so fight Oliver, get up and fight back!”
Arrow Season 5 “Kapushion”: “I WANTED TO!! AND I LIKED IT!!!”
Arrow Season 5 “Lian Yu”: THE ENTIRE EPISODE but probably the moment the flashbacks circled back to the pilot!
The Flash Season 1: “ Out of Time” When Barry time travels for the first time!
The Flash Season 1: “Fast Enough” THE ENTIRE EPISODE but probably the moment that Eddie SPOILER…
The Flash Season 2: “The Fury of Firestorm” King Shark’s reveal and Harry from Earth 2!