I guess I should just go ahead and get this one out of the way. Any discussion of classic Warner Brothers cartoons invariably turns to Chuck Jones' One Froggy Evening (1955), the surprisingly cerebral tale of greed, human nature, luck, and a singing frog. Steven Spielberg referred to One Froggy Evening as "The Citizen Kane of cartoons," and for good reason—it's not just a great cartoon, but stellar filmmaking as well. Note that this cartoon features not one single word of dialogue, only animation and Milt Franklyn's inspired soundtrack.
If for some reason you're unfamiliar with this masterpiece, it concerns a demolition worker who finds a metal box in the cornerstone of a decades-old building he's destroying. Inside the box is a frog who dons a top hat and sings various Tin Pan Alley hits, ragtime songs, and opera arias. The rest of the cartoon follows the worker's descent into madness as he realizes that the frog only performs when he's watching (or does it?).
Oddly, YouTube used to host a much higher quality version of One Froggy Evening as recently as a week ago; the only full-length version I could find now doesn't allow embedding. Perhaps Warner Brothers is having the cartoon removed. I managed to find a decent version on something called TubeMotion, which I've emedded here. Enjoy it here while you can. You can also find One Froggy Evening in the Looney Tunes Golden Collection, Volume Two DVD set.