This is based on opinion. Please don't list it on a work's trope example list.
Tear Jerker / The Sword in the Stone
The opening scene alone is enough to make you weep. The song in the beginning is so melancholy...*sniff*
A legend is sung of when England was young
And knights were brave and bold!
The good king had died and no one could decide
Who was rightful heir to the throne.
It seemed that the land would be torn by war
Or saved by a miracle alone!
And that miracle appeared in London Town.
The Sword in the Stone.
When Arthur and Merlin turn back into people after being squirrels and you hear the female squirrel who fell in love with Arthur sobbing hysterically. Man, it's hard to even think about without tearing up.
Merlin: Ah, you see, lad... that love business is a powerful thing.
In the same scene the saddest part is when Arthur just realizes how seriously heart-broken she is and he looks genuinely remorseful knowing he can't do anything to make her happy.
Enters into Fridge Tearjerker if you know how King Arthur's canon romance with Guinevere turns out. Then he'll really empathize with the squirrel.
This is even worse if you realize how different human biology is from other mammals. As far as humans go, Arthur is just a boy barely into puberty, but in squirrel form, thats well into adulthood. The female squirrel is already a grown woman, but Arthur cant return her feelings not just because he isnt really a squirrel, but because he's still a boy. He wont be ready for anything like that for years. We have no way of knowing if the girl was grief-stricken not just because she understood that her crush was human, but also because he was basically still a child and couldnt return her feelings if he wanted to.
Arthur's Heroic B.S.O.D. after losing the chance to become Kay's squire. Even Merlin felt sorry for him.
Which has nothing on how he feels after Merlin's reaction to his actually becoming a squire, especially when you realize that it's actually Arthur who's in the right here. After Merlin calls him out for not pushing himself to achieve bigger and better things, Arthur tearfully emphasizes (in so many words) that he can't— in the feudal era a child without a legitimate lineage had no prospects and almost no chance of getting ahead in life, and being a squire to a knight is the best chance that he can hope for. Or so he thinks.
Arthur: (tearfully bitter) What do you want me to be? I'm nobody! You... you don't know a thing about what's going on today! I-I'm lucky to be Kay's squire!
A brief moment during the wizards' duel when Arthur thinks Mim vanquished Merlin.
Arthur: Why that horrible old witch! I'll...I'll peck her eyes out!
Poor Arthur being saddled with the crown of England after pulling the sword out of the throne. He has nobody to guide him, aside from Archimedes, who has no idea how to help the poor kid. Arthur is a good kid with a kind heart, but has no idea how to lead, and his first instict is, understandably, to run away from his new responsibilities. Thankfully, Merlin reappears at the end to help him.
Arthurs reaction to Sir Ector bowing to him and begging forgiveness. Ector is, for all intents and purposes, Arthurs father, and while he was a strict, harsh disciplinarian, he was never abusive or cruel. He doesnt want Sir Ector (or Kay for that matter) to bow before him, because they are the only family he's ever known.
This gets even worse if you know the legend of Arthur. Arthur is King Uthers son (as far as anyone knows), but will never know his true parentage. Merlin isnt likely to tell.