It's a tie between "Rabbit of Seville" and "What's Opera, Doc?". Other favorites:
The Road Runner episode where the coyote puts fins and needle-noses on a bunch of dynamite sticks, and launches them from a hot air balloon, and for the rest of the cartoon, at odd moments, another "dyno-dart" comes buzzing in out of the sky and interrupts the coyote's latest scheme by blowing up.
Bugs and Yosemite Sam are in a mansion and Sam is trying to get Bugs out. He makes a trap door in the floor in front of the bathroom (which Bugs is hogging), and tries to get him to come out. Bugs stays put, and Sam gets frustrated and tries to break down the door, only to fall through his own trap into a valley about a mile below, cursing all the way. He storms back up into the mansion just as Bugs comes out of the bathroom, and tries to strangle Bugs, only to fall through the same trap door
again, cursing all the way down...
The episode where two mice slowly drive a cat insane, tormenting him and disappearing before he can figure out whodunit.
Gertrude and Heathcliff, two seagulls...No wait, that's an old Red Skelton bit!