This is based on opinion. Please don't list it on a work's trope example list.
Tearjerker / Only Fools and Horses
"Diamonds Are For Heather", the first depiction of Del's pining for a family. After taking to a young mother (whose husband had abandoned her) and growing attached to her son, he proposes to her, only to be turned down, her husband having returned asking for a second chance. It would be a long while before Del finally got to be a family man.
The scene where Rodney breaks down in the elevator after Cassandra's miscarriage in "Time On Our Hands".
A small moment in "Sickness and Wealth": after spending most of the episode in the hospital, Del finds out he only has Irritable Bowel Syndrome. He initially puts on a macho posture, claiming that he knew he was really alright, but after his GP leaves, Del breaks down in relief.
In "Little Problems", where Del is left alone after Rodney goes off to his honeymoon and new life. Simply Red's 'Holding Back The Years' playing in the background doesn't help...
The funeral for Grandad in "Strained Relations", and the part afterwards where Del puts his hand on Grandad's chair, now empty.
Rodney describing his feeling "cheated" every time he and Del remember their mother in "The Yellow Peril".
In "Who Wants to Be a Millionaire", Del turns down a very lucrative opportunity to move to Australia for a new business life, but decides to stay because Rodney is unable to go, and Albert does not wish to move. Rodney earlier accuses Del of being more interested in making money than he is in his roots, before Del launching into a passionate tirade about all the previous opportunities he's missed out on because he was busy raising his younger brother. Rodney thanks him, and Del telephones his partner-to-be to say the deal's off. It was the first episode to not end with a closing gag.