This is based on opinion. Please don't list it on a work's trope example list.
Tear Jerker / Power Rangers Wild Force
The Deer Zord thinks Merrick and the other Legendary Warriors abandoned him, so Merrick has to go to the Deer Zord and beg for his forgiveness at a time when the earth and the other Rangers urgently need his help.
Merrick: We didn't mean to leave you behind! [...] For three thousand years, I was in a tomb! I couldn't come and tell you! I'm sorry, Deer!
When Cole found out his parents are dead during "Secrets and Lies"
Cole: I came all the way out here...to find my parents. And all I have are these cold stones...
The deaths of the Wild Zords in the finale. Yes, previous seasons had their Zords destroyed in the final battle, but this time the Zords were living creatures who were being brutally killed by Master Org. Them all coming back to life in the end helps soften the blow.
The ending has the rangers saying goodbye to Princess Shayla as she takes their morphers, their jackets and the Wild Zords onto the Animarium nad and leave for the sky again. It is even worse for Merrick who basically begged to Shayla to let him come with her as it is hinted that there is something more between them then just protector and protected but Shayla had to sadly tell him that he can't come with her.
Out of all the Ranger teams, this one is the least likely to be reassembled, as Shayla took their morphers, their jackets, and all the Wild Zords onto the Animarium before departing to the sky again, implying that she will be returning to her slumber until a future generation of Wild Force Rangers is called. Although given their morphed return in the finale of Super Megaforce, that doesn't appear to be the case.
Although a proper reunion with the cast now seems impossible with the incident regarding Rick Medina.