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*
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.
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!