NOT “animation shorts”-please don’t post pictures of your Batman Underoos. 
I first saw this one at a Spike and Mike’s Sick and Twisted Festival of Animation many years ago.
Don Hertzfeldt’s “Rejected”
REJECTED by DON HERTZFELDT (Blu-ray restoration) - YouTube
Wow, that was truly horrific.
For me I think I would go with BASEBALL BUGS (1946)
It doesn’t appear to available so Baseball Bugs - Wikipedia
Everybody loves this one:
Your Face
Kiwi by Dony Permedi
Empty Roll
One Froggy Evening
Food Fight
Classic animation: Warner Bros.’ “What’s Opera, Doc?” (1957), starring Bugs and Elmer, doing Wagner:
https://www.dailymotion.com/video/x35sg3h
Modern animation: Pixar’s “Knick Knack” (1989), with soundtrack by Bobby McFerrin:
Bambi Meets Godzilla
I have a short attenti
Korgoth of Barbaria - which was briefly described as picked up for syndication, but sadly nothing came of it.
WARNING WARNING WARNING - NSFW (TV MA V)
For those who don’t want to watch the video.
Stop motion animation acceptable? I’ve always loved Balance.
La Maison en Petits Cubes (despite its French name, it’s a Japanese animation).
The Critic, starring Mel Brooks
(Back before anyone knew who he was. This is before The Producers, before Get Smart, even before he publicly performed The Thousand Year Old Man with Carl Reiner)
The Hangnail:
Frog Baseball, the original Beavis and Butt-head short.
Spoilered for graphic animal cruelty.
The Spirit of Christmas, the second South Park short, and the one that was directly adapted into the television show.
Spoilered for NSFW language.
People have suggested a lot of great ones, so I’ll recommend one that hasn’t been mentioned yet - The Mysterious Geographic Explorations of Jasper Morello.
Since I can’t figure out how to embed videos, I’ll just link it the old-fashioned way and say that “Duck Amuck” is my all-time favorite.