This is based on opinion. Please don't list it on a work's trope example list.
Tear Jerker / Honor Harrington
The horrible discovery of what happened to the survivors from HMS Madrigal in Honor of the Queen. Especially Courvosier's death, and Honor's bitter realization that she never told him she loved him. Some things just aren't fair... but oh, Honor, he knew. He knew.
Mike Henke bringing the news of Paul Tankersley's death to Honor in Field of Dishonor.
Honor's funeral in Echoes of Honor and her reunification with her parents in Ashes of Victory.
Mesa's attempt to assassinate Queen Berry in Torch of Freedom. In particular, Lara, realizing she can't get out before the nerve gas hits her, hurls Berry through a door where her lover, Saburo X, catches her. Lara has just enough time to say "I love you!" before Saburo closes the door.
The aftermath of Operation Oyster Bay in Mission of Honor, where not only was there damage to the Manticoran military, but debris from the attacked space stations fell onto Sphinx, wiping out much of the city of Yawata Crossing, nearly an entire colony of treecats, and most of Honor's family — including Andrew LaFollet, who died as he lived, saving Honor's mother and son. Some things are just. Not. FAIR.
The destruction of HMS Hexapuma during Oyster Bay. After everything that ship had been through... why, Weber?? WHY??? At least... at least her name was on the List of Honor. Terekhov's Hexapuma may be gone, but her legacy will forever live on.
In War of Honor, Hamish literally begging Honor to come back alive from Silesia, because no matter how impossible it is, he loves her and he can't bear to lose her again.
Sorrow Singer, the sole survivor of the Black Rock Clan of treecats all but wiped out by Oyster Bay, telling Honor that the Harrington clan has protected the treecats for centuries out of pure love — and now, the treecats will protect everyone fighting against Mesa for the same reason. Sorrow Singer felt her entire clan die thanks to treecat telepathy, and her sorrow should have broken her, but somehow, it didn't.
Sorrow Singer: We know how to deal with those who would slay those we love. Do not forget how Death Fang’s Bane and Climbs Quickly first met. They fought, and they bled, and each almost died for the other. Now it is our turn, and we wish you to go to Soul of Steel and Truth Seeker. Tell them the People — all the People of this entire world -— know who would protect them and who would slay them. We know how you and your clan have always loved and protected and shielded us from harm, Dances on Clouds. But the time has come for that to change, and we do not choose to be kittens forever. If you would guard us, then we will guard you, and if we die as Far Climber did, as Climbs Quickly almost did, as Laughs Brightly has almost died for you and you for him, then we will die. But we will not hide. We will not be children. If you will fight for all this world, for all of us, then we will fight for you.
From what we already know of treecat culture, that is not simply a Meaningful Name. When a treecat changes significantly, their name may be changed to match. Essentially she has dealt with her grief by becoming it.
Admiral Higgins having to nuke Grendelsbane, because he couldn't let what was there fall into enemy hands, and he had no way to defend the base with the force he had. His self-loathing is heartbreaking, and even more so because he had no choice at all.
It gets worse: Higgins is the Admiral commanding Home Fleet during Oyster Bay.
The entirety of "Last View" in The Shadow of Saganami.
"God damn it to Hell, Eddy!" Hargood exploded. "There are six of the bastards, including two battlecruisers! Just what the fuck do you think you're going to accomplish? Unlike us, you've got the legs to stay away from them, so do it, damn it!"
Then, after HMS Nike manages to destroy or cripple five out of the six enemy ships byherself, and despite massive damage:
"We're done, James," Saganami said. His voice was hoarse, harsh with pain and the exhaustion of blood loss, yet his expression was almost calm. "Tell the Queen. Tell her what my people did. And tell her I'msor—"
In At All Costs, Howard Clinkscales' funeral and the aftermath, when Honor creates the legal equivalent of blood bonds between her family and the Clinkscales family.
The end of the book, where Honor mourns the death of Alistair McKeon while reading to her children:
Wherever you are, wherever God takes you, fly high. I'll guard the Phoenix for you, I promise. Goodbye. I love you.
A page earlier, when at the end of the battle of Manticore, where millions of people have been killed, including many of Honor's friends and colleagues and then she gets confirmation that Alistair's ship has somehow survived and gets a good signal to the bridge. And then she learns that while the main bridge survived, flag bridge did not.
Not just Alistair. Everyone who died in the Battle of Manticore, on both sides... and it was all for nothing.
On the other side of the conflict in the same book, the death of Javier Giscard. One of the most sympathetic Havenite officers, who's been around nearly the whole series, and who's just gone through so damn much and survived, only to die because of the resumption of a stupid war that he knows happened because of an engineered misunderstanding, and killed by Honor herself, who had known him personally, and who also knew that the resumption of hostilities was on false premises, but can't do anything to stop it. All of it happening when by all rights, Honor should have been home, watching her daughter be born. Seeing his lover, Eloise Pritchart — the unbreakable President of the Republic, who has faced down two corrupt regimes and two coups d'etat and come out on top, who along with Tom Theisman gave everything she had to bring back the true Republic that had lain in ashes for two hundred years — so broken afterward just makes it so much worse. Suffice it to say that of all the deaths in the whole series — and good God, have there been a lot of them — this one might just be the most heartbreaking.
Honor is notorious for never betraying a single emotion, but the readers get to see it all from the inside.
The scene in The Short Victorious War where Helen Zilwicki's convoy (carrying her husband and four-year-old daughter) is attacked, and she almost doesn't say goodbye because no one in her crew will have the chance.
"The peeps won't get us, Baby. We're safe now. Mommy made it safe."
Even though she has Plot Armor, a young Helen Zilwicki Jr manages to get one of these when she resolves to lead a group of assassins far away from Berry and Lars, believing they'll be content just to kill her.
And so, in the end, Helen belonged to her mother also.
The entirety of "The Stray" from World of Honor. David Weber's not the only Honorverse writer who can break hearts. Just... fuck you, Linda Evans.
The senseless slaughter of a RMN shuttle by a mentally unbalanced crewmember of the Marianne / Golden Butterfly, in The Shadow of Saganami.
Pavletic, too. Mr. Weber is a cruel man. And it gets so much worse when Helen breaks down in Paulo's arms.
The end of The Shadow of Saganami when Hexapuma and Warlock limp back through the Junction, having foiled Mesa and prevented the war (briefly), and Home Fleet is waiting for them, and gives them the traditional salute to the Queen's yacht.