From "Doctor K": The entire episode, especially Dr. K's flashbacks to her life growing up at Alphabet Soup, counts as one giant tearjerker. Someone please give her a hug.
From "Ghosts": Dr. K's reaction to seeing Gem and Gemma for the first time since Venjix attacked Alphabet Soup. And she smiles, even with tears running down her face.
From "Ancient History": Dr. K's exchange with Colonel Truman in Corinth HQ.
Dr. K: We all do things in life we regret. But if we don't learn from them, and move towards a brighter future, we pay twice.
Colonel Truman: I've already paid more than most.
Dr. K: I know. And if I could access the space-time continuum long enough to undo what I did, I would. That's not possible.
The last few scenes of the series has two. One where they were returning their morphers and Ziggy quietly, and not just a little bit sadly, says "Get in gear". And the second where Dillon returns to a flower we saw in the first scene.