Mine is Superman vs Captain Atom

This is my favourite aswell

I got two:

Superman vs Captain Marvel

Flash/Lex swapping bodies.

Atom getting a free ride in Wonder Woman's cleavage.

Batman Vs. The Injustice League, that was some awesome mental kung fu.

Superman vs Amazo. Superman actually fights intelligently and *gasps* dodges attacks! Not to mention Amazo had the Flash's speed and the power of a lantern ring, making it quite impressive that Superman could hang with him.

That one where Etrigan and Batman team up and when Superman stops holding back against Darkseid come to mind.

Justice League Unlimited - Season 3, Episode 8 - The Return

Batman vs Doctor Destiny.  
  

1. Superman vs. Captain Atom 2. Superman faces off against Darkseid in the finale.

@Gambit1024 said:

I got two:

both of those fight were really cool...two of my favorites as well...

1. Flash vs Lex 
2. Justice League & The Secret Society vs Apokolips 
3. The Secret Society vs The Secret Society

When Superman stopped holding back.

And when Flash beats Lexiac or brainior.... nvm

@GrandSymbiote94 said:

When Superman stopped holding back.

And when Flash beats Lexiac or brainior.... nvm

Cool moments indeed.

Also the episodes Grudge Match and Clash.

@Gambit1024 said:

This is mine too

@EugeneSaxe said:

Flash/Lex swapping bodies.

Atom getting a free ride in Wonder Woman's cleavage.

Dr. Polaris: "Aren't you going to wash your hands?"

Lex Luthor: "No... Cause I'm evil."

Lol, Flash is such a troll.

@Gambit1024 said:

I got two:

THIS.

Tie between Flash taking down Brainiac amd Wonder Woman kicking the asses of most of the female members of the Justice league at once

I really liked the fight between Batman and Batman, in the episode with the Justice Lords. Can't remember what season or episode that was. It's not very epic, but it touches on some emotional chords.

1. Superman unleashes his full power over Darkseid

2. Flash goes berserk on Luthiac

3. Superman vs. Captain Atom

4. Superman vs. Captain Marvel

1. Wonder Woman vs Justice League
2. Superman vs Wonder Woman
3. Superman vs Captain Marvel
4. Superman vs Darkseid both times
5. Braniac vs JL
6. JL vs Justice Lords
7. Supergirl vs Galatea
8. Grundy vs JL and Doctor fates gang
9. Martian Manhunter vs The Anihilator
10. Justice League vs Manhunters

Superman vs Captain Atom, Green Lantern vs the Manhunters, and Green Arrow vs the giant robot in "Initiation"

The entire episode "The Return" and I think it was called "Panic in the sky" both were nice work

"Task Force X"

There's a brief fight between the Task Force and Shining Knight, Vigilante, and Atom-Smasher. While it's not as 'explody' as the other fights, I really liked this close-quarters engagement. It was placed right at the climax of the episode, making it all the more nail-biting, and there's one awesome bit when the Annihilator is about to step on Vigilante, before Atom-Smasher literally lifts the Annihilator from the foot-up!

"Wild Cards"

Quite possibly the best episode from JLU, the entire episode is just one extended fight scene. The intense chase for the bombs, the overall fight with the egotistical Royal Flush Gang, and even Batman's final confrontation against Joker and Ace. Really great moments here, and one of McDuffie's best.

Superman vs. Captain Atom and Green Arrow vs. Wildcat.

Superman vs. Darkseid (from Season Two - Twilight of the Gods) - it helped me get over the ending of Superman: TAS.

Man, looking at all these replies.. i miss JL/JLU. really one of the best, if not, THE best, animated shows ever. Spectacular writing to say the least.

Superman is not holding back.

Flash vs. Luthor