During "Legendary Catastros" watching Nick get over his fear of Catastros and managing to help the wounded beast.
In "Ranger Down", Phineas finding and adopting an abandoned Fireheart, it's best when Fireheart starts calling him "Daddy".
Jenji staying well-past his time limit to try to help the Rangers against the Monster of the Week and risking vanishing forever.
Madison: Jengi, why didn't you go back to Rootcore and save yourself? Jengi: You guys are my family... my only family. Tell Fireheart I'm sorry.
Near the end of "Heir Apparent Pt. 2" when Udonna reveals to Koragg/Leanbow that he's Nicks father, and her husband. Koragg stands their stunned for a second, then in a moment of clarity reaches out to his son. It's cheesy, it's Narmish true, but it's still a very heartwarming scene.
"Mystic Fate Pt. 1", after the master has possessed Nick, he forces Nick to don Koragg's armor and attack innocent creatures. Eventually Nick is talked down by his father much in the same way Udonna did for him, this manages to break him free of the Master's Mind Control.