"Death Punchies" gives us a whiff of 80's epic as early as the fourth episode, in the form of a short montage set to "You're the Best Around".
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 ending theme that doesn't air on Cartoon Network proper, but does play when shown online and on DVD and Blu-Ray.
Kenny Loggins' "I'm Alright" proves to be a perfect fit for the opening of "First Day", in which we see Mordecai and Rigby getting their jobs at the park and moving into their room.
Pops' awesome rendition of "Footloose" in the climax of 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". The Disney Acid Sequence that ensues is excellently crafted and fits the song quite nicely.
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.
The background music during "Think Positive" when Benson builds up too much rage and starts to turn red all over and float in the sky, turning into a mass of power that threatens to destroy the entire park. The music sounds like the Bruce Faulconer soundtrack for Dragon Ball Z, and is not only fitting, but intense and epic. Full scene here.
"The Christmas Special" has Trans-Siberian Orchestra's "Christmas Eve/Sarajevo 12/24", a rock-remix of "Carol of the Bells". And it is awesome.
Mordecai and Rigby's song in the Thanksgiving special is a cross between awesome and heartwarming. The episode's opening and ending theme are also worth putting on this page.
The Metallica-esque thrash metal song that underscores the scene where Benson, Mordecai and Rigby are driving down the world's deadliest highway in "Busted Cart".
Party Tonight. Who cares if they were lip-syncing, it was still an exceedingly awesome song! CN made an awesome 80s music video for it. It's live action, and it has lasers.