Any episode from any DCAU show and why?
Too many great episodes to pick from to be honest. Though it's not my #1 favorite episode, the episode "The Doomsday Sanction" (Justice League Unlimited) I thought was always a great episode. Superman fights Doomsday at the heart of a volcano while the Justice League quickly try to save the people before the volcano explodes. But the best part of the episode is Batman stopping a Kryptonite missile which is followed by this scene...
The fact that Batman can say that he literally "took a bullet" for Superman, to me, is something we don't see everyday and just makes the episode feel even more serious than it already was.
Perchance to Dream. It's the 26th episode of Batman: The Animated Series. Hands down it is the best episode of any series in the DCAU.