I was surprised at how sad the Snow Lion ep from '63 was, especially that scene with all the robots in the museum telling Dr. Ochanomizu not to give up.
Someone else mentioned the Damdam's Head ep which I thought was really good. Also a really sad one from '80 was Astro vs. Atlas: Atlas forever. Like wow, so many people died in that episode. And just how distraught Astro was throughout most of the ep
![Crying or Very Sad :cry:](./images/smilies/icon_cry.gif)
And not super sad ep, but a touching one was the Robot Circus from '03.
And of course all the Pluto eps killed me.