Sgt. Frog/Tear Jerker


 * The Season One Finale (and the manga equivalent, "The World's Smallest Invasion") is probably one of the biggest. It would be hard to come up with a more painful way to retouch upon everything shown in prior episodes.
 * The first movie, all the human/Keronian partners breaking up and everyone's subsequent reactions. *sniffle*
 * Keroro and Alternate Universe Fuyuki parting ways in episode 205. A surprisingly, and rather strikingly good, tearjerker moment.