Brenda takes Nate and David to the bus that caused Nathaniel's death, where the two brothers come to (better) terms with father's passing.
Cutting straight to the heart of the show's message: Nate comforting a grieving woman:
Grieving woman: Why do people have to die?
Nate: To make life important.
The entire show's "message" distilled into a few lines of dialogue. Heartwarming, Awesome, and thought-provoking:
Nate Sr.: You can do anything, you lucky bastard, you're alive! What's a little pain compared to that?
David: It can't be that simple.
Nate Sr.: What if it is?
In David's last (imagined) confrontation with season four's hitchhiker/hijacker, he attempts to deliver 'the final blow' in a dream before seeing himself in the red-hooded figure. He then embraces his other self, presumably coming to terms with his fears, and wakes to find a vision of Nate smiling over him. The ensuing, reassuring silence is particularly heartwarming.
Nate's relationship with his daughter Maya. In particular, the moment when Maya goes to visit Nate in hospital right before he dies and cuddles up to him is very sad and very sweet.
"To Nate." The Fishers have a toast in Nate's honour when they all have dinner together.
A moment which combines Heartwarming Moment and Moment of Awesome: the last scene, which cuts between Claire driving off to New York City and flash-forwards of all of the deaths of the surviving main characters. It's surprisingly heartwarming in context.