Boris trying to convince Balto not to go after the missing sled team all alone: "But who's going to tell you how cold you are?"
And Boris saying the film's Arc Words: "Let me tell you something, Balto. A dog cannot make this journey alone. But, maybe a wolf can."
Jenna lying on the sick Rosie's bed, comforting her.
Sylvie telling Jenna and Dixie she heard a rumour that Jenna's dating Balto, the town outcast. Jenna's response is basically, "Yeah, I am. (You got a problem with that?)" Given how much the other dogs ostracize Balto, this is quite sweet of her.
The end of the first film counts as this. After everything he's had to endure, Balto finally gets the reward he deserves.
The flashback sequences of Balto bonding with his pups in both of the sequels are pretty touching as well. Wings of Change in particular shows Balto helping Kodi out as a pup when the little guy kept getting left behind by his siblings.
Balto and Aleu making amends at the end of Wolf Quest, assuring each other that while this is goodbye, they will never forget one another. Doubles as a Tear Jerker.
On a similar note, there's also Kodi coming to Balto's aid in the climax of Wings of Change. Initially, Balto is the only one wanting to help the pilot after he crashes, as Kodi and the other sled dogs were willing to let him die so he wouldn't take their jobs. However, Kodi has a change of heart following a What the Hell, Hero? speech from his mother and manages to convince the team to help his father save the very man that threatens their job. It is particularly heartwarming as Balto is about to slide off a crumbling ledge, only to be then caught and saved by his son.