Black Jack: I don't know who you are, but I don't think you know what being human really is. Humans can be weak... yet because of this, we can feel kindness and sorrow. That's what makes us different from robots.
Dr. Tenma: Astro, you must exceed the limitations that humans cannot! You must go beyond nationality, ethnicity, philosophy and religion, starvation and poverty, and war! Exceed these limitations created by man! You must fly high! Higher than any human has ever flown before! To reach for the future that no human has been able to so far!
Astro Boy's refusal to let Nuka die alone. "I'll always be with you."
And after so much art and fanfiction by sad fans who desperately wished her first incarnation had lived, we actually got an adaptation where she can! Tell me you didn't squee while seeing Atom trumphantly flying back to Earth with Nuka in his arms?
The revival of Prime Rose and her reunion with Daichi, Who was actually Atlas with his formerly human mind implanted; Doubly so as her revival from years in cryo-sleep comes as her father Drake is about to murder Duke Red, only to learn that his daughter had never been kidnapped by a robot.
Drake's final request before giving himself up:
Drake: Daichi? Take care of my daughter for me.
Unico granting Astro's wish to talk with his father.
The entire game. Seriously, it reads as one long love letter to Osamu Tezuka and his creations.
The end of the first half, despite looking like a Downer Ending; after the credits roll, the Phoenix revives Astro and gives him a second chance. What really makes this work is that this is Tezuka's final creation meeting his most famous.