That epic chiptune music that plays during the battle against The Destroyer of Worlds.
"Don't look at our crotches while we synchronize our watches!"
The Thompson Twins "Lies" montage when Mordecai and Rigby try to out lie each other
Mordecai's singing in "Mordecai and the Rigbys".
The background music when Mordecai and Rigby are being watched by hundreds of cameras that Benson set up.
Mordecai and Rigby's party snack rap.
The Battle of the Ear Worms, along with Benson's epic drumming skills.
Mordecai and Rigby doing a Training Montage on an arcade game while "Hangin' Tough" plays over the sequence. Seriously, the writers seem to love dated 80s music. It's pretty awesome that the producers are willing to pay for all this music, since the licensing fees must be huge.
The score piece that plays during the audit in "Don". It almost sounds like it belongs in a theatrical production.
The use of "I'm Alright" over the opening montage showing Mordecai and Rigby getting their jobs at the park and moving into their room.
Pops singing "Footloose" in the karaoke episode.
"Hey Man, Nice Shot" by Filter playing during the climax of "Slam Dunk" (even though that song isn't about basketball playing; it's about the suicide of a corrupt politician that was broadcast on live television).
"Mississippi Queen" by Mountain plays after the Mordecai, Rigby, and Benson imbibe a concoction of the same name during the climax of "Weekend at Benson's".
Mordecai has a training montage to "Holdin' Out For A Hero" so he can eat a 12-egg omelette in under an hour in "Eggsellent".
Dies Irae plays when Skips beats Rigby in arm wrestling and kills him.
"Ballroom Blitz" during the party-off in "TGI Tuesday".