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.
"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).
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.
Party Tonight. Who cares if they were lip-syncing, it was still an exceedingly awesome song!