I am HERE for this topic.
I'm loser for well done animation
let's see what I can share that hasn't already been posted
The Cat Piano
A short that will leave you unraveled and curious. The animation direction is simple but intense in some areas and the story fascinating on top of fantastic narration. Reminds me of old school Noir. A bit unnerving in context just as a warning.
Pretty much anything from the school Gobelins
This is a student film and it isn't the only one of this caliber. This one in particular is a tad nsfw just as a heads up!
Rise of the Guardians
Got heavily overlooked and there was a massive drama fit over this back in the day. Storywise can be a bit wishy washy but the animation is extremely detailed and stunning. If you want snow that is insanely better then Frozen - Watch this.
Sinbad: Legend of the Seven Seas
Old school Dreamworks back when they were finding their footing with 2D mixed with 3D. A bit cheeky due to that but worth it just from the animation on the villian's hair ALONE. Look at this shit and tell me otherwise.
Mune
Plot is a tad generic! However the art direction makes me cry.
Rock and Rule
Need a very clearly 80s film? Here you are. Pink Floyd's the Wall meets 80s fantasy and sci-fi full force. The trailer is a mess so have this instead.
Secret of Nimh
I'm startled there's not more Don Bluth up in here. He's probably the reason behind the disney renaissance [it's theorized at least].
An American Tail: Fieval Goes West
i actually liked the sequel more as a kid then the first whoops
Summer Wars
An all time favorite. Same creator of Wolf Children so his style is apparent.
I've got plenty more recommendations, including some old school Asian films and such, but I think I've posted enough for now.