When the trio have to take Mushy/Tarax/497 into outer space for it to explode harmlessly lest it destroys the Earth. After doing so they find that the explosion created a new constellation and when asked to pick a name for it. They combine their respective names for the creature, giving us the episode title.
The battle sequence, with Octus being energized by the power of love, as actual '80s pop ("Space Age Love Song" by A Flock Of Seagulls) is playing. Also, interspersed with shots of his new girlfriend dancing down the street, listening to the song.
For that matter, Octus' entire motivational speech to Kimmy stating that she's not "stupid" and that she just needs to learn how to apply herself to her studies.
Kimmy: Y'know, no one has ever talked to me this way before. Octus: Well, it's about time someone did.
The end of "The ballad of Scary Mary" where we find out what happened to poor Mary. She was picked up by a rather handsome, motorcycle riding guy, and rides off into the night with him.
The ending of Episode 13 when Ilana recovers after her Disney Death, and Lance and Octus are relieved that she is all right.
"I am Octus."
Lance's dance with Ilana at the end of "Under the Three Moons."
The group picture shots right before the prom as well. It shows how far along Kimmy has come seeing how doing all the silly poses with people she previously considered losers. Sadly this led to the eventual Tear Jerker.
Episode 18 and its flashbacks showcasing the trio growing into real True Companions.
In a way, Lance's Unstoppable Rage moment at the end of episode 19 when the monster kills Octus, as it shows how far they've come as companions since Lance is that angry at Octus' death.
The entire last-third of episode 20.
Also, earlier in the episode when Kimmy and their other friends show to be very worried for them. At the end of the episode, Octus sends Kimmy a message telling her that 'he'll see her soon', causing her to tear up and smile.