In "Hell Week" (2x10), after Rusty sneaks into the Omega Chi house to steal the pledge paddle with which he beat Evan in the pilot, Evan spots him, which prompts Rusty to run like hell, chased by the entire frat house. His screams are heard by his fellow Kappa Taus, which run to his aid- Evan manages to tackle Rusty, just as the two frat houses collide head on in what is by far the most spectacular brawl that has been seen so far on the show.
Don't forget they made it rain beer in "Liquid Courage" (1x05).
The catfight in "Our Fathers" (3x02) is pretty funny.
Drunk Rebecca in "Spring Broke" (1x22).
The dance sequence, burning down a rival house accidentally, destroying the security guard's golf kart in "Friend or Foe" (3x09) are all pretty awesome. Also Jeremy got his first and last line of the series.