This is based on opinion. Please don't list it on a work's trope example list.
Tear Jerker / Xena: Warrior Princess
Marcus' death and funeral in "The Path Not Taken".
It gets even more heartwrenching in "Mortal Beloved", in which Hades agrees to restore Marcus back to life, but only for 48 hours, so that he and Xena can retrieve Hades' stolen helmet (without it, he is nearly powerless). When Marcus is nearing the end of his second chance at life, he and Xena kiss, before she plunges a dagger into his heart. Then she tearfully pleads with Hades to send Marcus to the Elysian Fields, rather than allow him to spend eternity in Tartarus. He judges Marcus as worthy for the Elysian Fields, but of course Xena can't stay... cue another emotional goodbye. *sniff*
Gabrielle: You'll be together again one day.
Xena:(fighting back tears) We'll never be apart. (holds her fist to her heart) He's in here, forever.
Xena trying to revive Gabrielle in "Is There A Doctor in the House?". Of course, Gabrielle gets better, but still:
Gabrielle's death at the end of "Sacrifice, Part II".
The ending of "Death in Chains", when Celesta takes Talus to the Underworld while Gabrielle looks on, absolutely devastated.
It gets even more heartwrenching when she collapses into Xena's arms, sobbing against her chest.
"The Greater Good". Believing Xena to be dead, Gabrielle manages to hold herself together for a little while... until she goes out to the woods and starts beating a tree with her staff, screaming and crying.
Believe it or not, even Salmoneus has his moment in this episode. He's usually such a goofball, but his quiet display of grief upon finding Xena apparently dead on the floor definitely tugs at the heartstrings.
Xena finding her son Solan dead at the end of the second act of "Maternal Instincts". Her screams of absolute anguish are heartwrenching.
She denied it at first and then cradled the teenager's body like a baby as the truth started to sink in. You could see it coming, so you'd think it wouldn't be that sad. But then this line comes up:
"Solan. Solan, please. I'm here now. Your mom is here now just like you always wanted."
Despite being a very bizarre episode, "The Bitter Suite" had its moments such as Xena and Gabrielle's confrontation in the Hall of Echoes.
Xena: It echoes whenever we talk about the past. Tell me something not in the past!
Gabrielle: I hurt inside! Don't you?
Also from The Bitter Suite, is Xena's song to Solan and Gabrielle, asking them to forgive her for lying to protect them, especially the half to Solan. One can see Xena's trying so hard not to break down sobbing, and she wants their forgiveness so bad. It's such a touching song, too.
The end of "Forgiven" when Xena refuses to undergo Apollo's ritual of forgiveness and just walks away.
Hope and her son The Destroyer dying in each other's arms was sad. They did an excellent job of giving it an air of tragic pathos.
The end of A Comedy Of Eros. The look on Joxer's face when Gabrielle laughs off the idea of him and her getting together. Innocently Insensitive much, Gabrielle! After she walks away, Joxer gives a very bitter and sarcastic "ha ha." Xena places a hand on his shoulder in a gesture of comfort, and as she walks away too we can see Joxer about to cry. He may be an annoying goof-ball at times, but his serious moments are truly serious.
Strangely for a villain, Callisto has a fair number. The moments when she isn't cackling with malicious glee, where she gives a glimpse of the damaged child she is underneath her malice, can be genuinely jarring and sad.
From "Return of Callisto", when she's about to burn Gabrielle at the stake, she says with a tone of longing: "I envy her in a way. She gets to leave this world so pure. I wonder if I could have been her?" A few seconds later, Xena struggles against the bonds of Callisto's former prison chair in a vain attempt to save Gabrielle, and Callisto turns to her with a look of exasperation: "Oh don't struggle, Xena. Be thankful for that beautiful throne! [softly] It'll give you peace if you let it." The way her tone lowers as she says the last part, with no taunting or malice... It's as if she's talking about pain, loss, just simply giving into the prison of it, can give peace.
During the events of "A Necessary Evil", Callisto has Xena go to a small village and publicly declare her crimes against Cirra, Callisto's home village. The whole scene is one huge tearjerker - we can see Xena's discomfort and remorse for what she did when she was her former evil self, and gradually Callisto's face goes from satisfaction at Xena's discomfort to looking as though she's about to burst into tears. When Xena comes up to her, Callisto gives her a look that's sad and almost pleading, which Xena cuts short by saying, "Let's go." Even after the others have walked away, Callisto stands there for an extra moment, still with that look of anguish on her face.
Specifically, Callisto's face looks the same as it did when she was a child, which is an interesting callback. Two short episodes ago, in "Destiny", Xena visited Cirra as a way of facing her guilt for her past crimes. Among other flashbacks of the burning village, she saw the face of a tear-stained child Callisto, asking "Why?" The look Callisto gives Xena when she confesses her crime mirrors her child self's look exactly, except now she's too broken to cry or ask "Why?"
Later Gabrielle asks her if she felt anything when Xena apologized for her crimes. Very solemnly, Callisto answers that she feels bits and pieces of emotions, but nothing solid. Then she says this: "Think back to when you were a little girl, and all you knew was your mother and your sister, and all your faith revolved around them. Now kill them." Gabrielle is in tears as her words sink in.
Then there's the episode when Callisto dies, again. After Xena stabs her, she doesn't flop over, nor does she try to get vengeance on Xena with her dying breath. No, instead she caresses Xena's face as she falls, as if she's saying 'thank you' for finally ending her suffering and putting her next to her family in death.
Then later this continues to be heartbreaking when, as an arch-demon, Callisto screams at Angel!Xena: "I will never stop hating you, Xena! Do you hear me? Never! You killed my family, my soul, my reason to love!" And then to make it more tear-jerking, Angel!Xena whispers, "No..." and gives Callisto her light, thus forever curing her from her agony.
"A Good Day" ended with Xena & Gab saving a village from Caesar and Ponpey's armies but many people died in the process.
"The only thing we can take from this is that it's been a good day of fighting".
The end of "Destiny".
Most of the Xena/Gabrielle scenes from "One Against an Army"
The crash scene and aftermath of "Looking Death in the Eye"
The poem scene at the end of "Many Happy Returns"
The two "Friend in Need" episodes, especially the bit around sunrise of part 2.
Gabrielle at the end of "The Deliverer." Forced to kill and used in a horrific ritual. "Everything's different now."
So much of it in "Orphan of War." Xena gave up Solan during her warlord days both because he would be a target and because he would turn out like her. Years later, she encounters her son, who believes she murdered his father and is still a monster. Though those opinions are corrected, the episode ends with Xena not telling him the whole truth. It's the right thing to do, but you can see how much it hurts in that last shot.
Toward the end of "Coming Home," when Ares thinks he's just killed Xena, his reaction is just brutal to watch.
Zeus's death in God Fearing Child. Even as Hercules kills him he acknowledges his fate and even tells him that he's always been proud of him. Even when Hercules kills him he expresses pride: "Hercules, you have never disappointed me, especially today." Hercules is in tears as his father dies.
And before that, Hera's death at Zeus's hands. Her husband, who caused her so much pain over the years by cheating on her and betraying her, is the one who kills her, with a kiss no less. He kills her for helping Hercules, one of the many children who came from his affairs, whom Hera has hated for most of his life. The final conversation between Hera and Hercules is a tearjerker in itself:
Hercules: I never thought I'd say this. I believe you. But how can I be sure you aren't gonna change your mind?
Hera: Because of a lesson I regret teaching you long ago, Hercules. Parents should never outlive their children.
Xena's moment of lucidity in 'The Furies,' after Gabrielle finds her, naked and screaming about how the women and children of a village have been crucified - when the only people in the village ARE women and children.