This is based on opinion. Please don't list it on a work's trope example list.
Tear Jerker / Lois & Clark
In "Man of Steel Bars", Clark has no choice but to leave Metropolis because he can't take not being allowed to use his powers to help people while beginning to believe he's the cause of the intense heat wave. Later, he tells Lois he's leaving the Daily Planet. Lois believes that Clark idolizes Superman and feels just as bad as he does, and she tries to talk him out of leaving, but Clark has made up his mind. He kisses Lois and says "goodbye."
Even Cat Grant tries to comfort Lois.
"That Old Gang of Mine":
When Clark gets shot and has to play dead to preserve his identity, Lois begins having a total breakdown, sobbing as one of Al Capone's goons takes away the body of her best friend. Later, she's shown walking through the Planet in a daze in old, ragged clothes. It's like her whole world's been taken away.
There's a subtle moment when Clark is shot, he doesn't immediately fall over and play dead, but a brief moment of Oh, Crap! as he realizes his life as he knows it is over.
Jimmy's tearful moment where Perry comforts him and assures him that yes, it's okay to be sad, and yes, things are going to be okay.
There's also Clark's talk with his parents about how his life as he knows it is over.
Clark: Dad, Superman doesn't work at the Daily Planet. He doesn't go to ballgames with Jimmy and Perry. He doesn't listen to Lois go off on some weird tangent and secretly love it.
Mayson's murder in "Lucky Leon." And the last thing she learned is Clark (the man she loves) is Superman (the man she dislikes).
In "Home Is Where the Hurt Is", Superman is sick for the first time in his life, and Ma and Pa Kent are terrified for him.
The death of Lois' clone.
Lois meeting alternate!Clark, who's bullied by Lana Lang, never became Superman, and had his Lois disappear.
In "The Night Before Mxymas", Mr. Mxyzptlk traps the world in a time loop where each time around, humanity would grow more depressed and pessimistic until they all crossed the Despair Event Horizon and destroyed themselves. An elderly man Superman saved from dying is grateful the first time around, but after a loop or two, he wishes he hadn't been saved.
Possibly even worse is the scene where Mxyzptlk causes Jonathan Kent to have a heart attack.
The very end of Season 3, where Lois is left all alone on Earth, apparently forever.
Perry's sad little talk with Lois, Clark, and Jimmy after he learns that Senator Black, a man he's known and admired since he was Jimmy's age. Was arrested after being outed as a Nazi sleeper agent. Commenting that he's going to take the rest of the day off.