The Academy of Motion Picture Arts and Sciences defines a short film as "an original motion picture that has a running time of 40 minutes or less, including all credits."
Film Critics Pick The Best Short Films Ever Made, via IndieWire:
25 Short Films to Watch on YouTube, via Ranker:
16 Short Films That Launched the Careers of Famous Directors, via Mic:
Best Short Films, via discover.film:
(Moved this list to Letterboxd https://letterboxd.com/trinisete/list/all-animated-movies-i-have-ever-seen-from-1/)
I will keep updating this list, so the number will always change.
Last addition: Wonder Woman
You need to watch these in order.
Aladdin > Aladdin and the King of Thieves (You can skip The Return of Jafar and the series is optional)
Cloudy with a Chance of Meatballs > Cloudy with a Chance of Meatballs 2
The Garden of Sinners (Kara no Kyoukai) | Overlooking View > A Study in Murder (Part 1) > Remaining Sense of Pain > The Hollow Shrine > Paradox Spiral > Oblivion Recording > A Study in Murder (Part 2) (Prologe and Future Gospel are optional)
Mobile Suit Gundam I > Mobile Suit Gundam II > Mobile Suit Gundam III (You can skip the first anime, than watch the series in this order: Zeta Gundam > ZZ Gundam > Char’s Counterattack > Gundam Unicorn. Origin is a prequel to everything, but can be watched later)
These are better if you know the original source.
The Adventures of Tintin - The Adventures of Tintin (Comic or Cartoon)
Batman: Assault on Arkham - Batman or Suicide Squad (comics)
Cowboy Bebop: The Movie - Cowboy Bebop (Anime)
Dexter's Laboratory: Ego Trip - Dexter's Laboratory (Cartoon)
Digimon Adventure: Our War Game - Digimon Adventure (Anime)
Highlander: The Search for Vengeance – Highlander (movie)
The Princess and the Robot – Monica’s Gang (Comic)
Professor Layton and the Eternal Diva - Professor Layton (Game)
Ratchet & Clank - Ratchet & Clank (Game)
Scooby-Doo on Zombie Island - Scooby-Doo (Cartoon)
The Simpsons Movie + Maggie Simpson in The Longest Daycare - The Simpsons (Cartoon)
Space Jam – Looney Tunes (Cartoon)
The SpongeBob SquarePants Movie - SpongeBob SquarePants (Cartoon)
Wonder Woman - Wonder Woman (Comics)