The first moment comes from "Whatever Happened To The Man Of Tomorrow?" by Alan Moore and Curt Swan (with George Perez doing inks.) This was supposed to have told the final story of the Silver Age Superman before his series and history were rebooted. These two pages are amazing to me - in particular because of Krypto staying by Superman's side as he cries. (Another moment you can vote on would be from the same story, with Krypto's sacrifice to save the people he loves, but....well, no we don't need to have that one on here, do we? No, I think not....)
The SECOND moment is from Crisis On Infinite Earths #7 written by Marv Wolfman and art by George Perez (my all time favorite artist). The death of Supergirl made headlines around the world and I remember when this came out back when I was 13 or 14 this was an impossible issue to find. I eventually found it when I was in my late teens and if memory serves, paid a small fortune for it (well, a small fortune to a mall rat like me.) I think the panels that are the most emotional to look at would be "Oh, no," the small panel of Captain Marvel (aka Shazam!) wiping away a tear and the last panel after Kara dies. It's melodrama at its finest, but I love me some melodrama!
If you're a comic fan like I am....and I know there's a bunch of you out there, take a look at this poll and see if anything stands out to you....but do yourself a favor, bypass those "Death of Krypto" pages...they still get me every friggin' time.