This is based on opinion. Please don't list it on a work's trope example list.
Tear Jerker / Spider Riders
Sparkle's reaction to Grasshop's betrayal in "Trials of the Prince's Group"/"Grasshop's Misfortunes". Sparkle keeps giving Grasshop a hurt look. It's so sad that Grasshop can't even bring himself to look her in the eye. It's no wonder Grasshop wanted to make things up to her so badly that he did a Heel–Face Turn for real later on.
Sparkle: (confused) Why are you doing this, Uncle?
Grasshop: (Grasshop looks at Sparkle, but turns away when he can't bear to look at her) *I wish she wouldn't look at me like that! She is my enemy! So why should I care how she feels?*
Buguese in "Parting and Meeting Again"/"Reunions". Though what he does to get back Aqune is extreme, you can clearly see why he does it in how crushed he is over being separated from Aqune. Even when he puts the mask on her, he doesn't look at all happy about it. It's one of those times even he can't hide how much he cares about her.
Poor Grasshop in "Someone to Believe in"/"Spies and Lies", having to choose between betraying the Spider Riders, or never seeing his wife and children again.
When Stags appeared to commit suicide after Hunter had beaten him in single combat. Hunter desperately reaching for him makes it all the worse.
Watching poor Beerain collapse and cry in despair at the end of "Flightless Butterfly"/"Fly No Longer". That scene alone is enough to make just about anyone feel sorry for her.
That's nothing compared to the way Buguese treated her when she finally did manage to catch up to him. You could tell that his being forced to disown her in order to stay in Mantid's good graces was eating him away inside.
Watching everyone's reactions to losing sight of Hunter and Shadow after their fall from the bridge in the Labyrinth in episode 45. Corona keeps shouting into her Manacle while hoping for a response. Magma is furious, Lumen just sits and sulks, and a devastated Grasshop tries to comfort the crying Sparkle.
In "The Emperor's Trap"/"Mantid's Trap", when asked why he didn't step up against Mantid sooner. Quake reveals that many of his fellow Spider Riders died during their last encounter, and he didn't want to repeat the experience.
The apparent death of Buguese near the end of the series. Even if you loathed him up to that point, you can't help but be impressed with his determination.
Hunter: Goodbye Buguese...
Even though he'd been a complete douchebag for the entire series, Mantid's story of the loss of his lover really gets to you.