2. "The Death of Jean Grey!" by John Byrne, Chris Claremont and Terry Austin (X-Men #137)
So Jean Grey had her mind warped to the point where she becomes Dark Phoenix and wipes out a whole planet of beings. The Shi'Ar decide to get involved and the X-Men are forced to fight the entire Imperial Guard to save their friend's life, who they feel is back to normal after Professor X placed a sort of psychic cap on her after the team fought Dark Phoenix themselves.
They're going along, getting their collective ass kicked by the much larger and more powerful Imperial Guard (but let me note, they're so awesome and so powerful but they seem to, like, never win a fight) when suddenly the blocks Xavier placed on Jean start to crumble and Phoenix returns!
Her teammates must now turn on her...
but ultimately, Jean decides that she cannot control the Phoenix force so she makes the ultimate sacrifice...
It's funny - the moment is awesome and unexpected (star characters in popular comic books just did NOT die in comics at the time, even in a title like X-Men where a member was killed off in their second mission) but looking back, Cyclops' exposition-heavy speech after the love of his life just incinerated herself is kind of funny.
This is the highest ranked moment on the list to later be retconned. It was later revealed that the Phoenix force was an independent cosmic being that made a deal with Jean way back in X-Men where it would save Jean's friends in exchange for Jean letting it take Jean's place and live life as a human being. So its "corruption" was really it losing traces of its copied humanity. So according to this retcon, it makes the sacrifice almost MORE noble, as it is one thing for Jean Grey to sacrifice herself for the good of mankind, it's a whole other thing for a COPY of Jean Grey to sacrifice itself for the good of mankind.
1. "The Death of Gwen Stacy!" by Gerry Conway, Gil Kane and John Romita (Amazing Spider-Man #121)
Kurt Busiek and Alex Ross made a point of framing the death of Gwen Stacy as the figurative end of the Silver Age, and it certainly does have that sort of feeling to it. It's such a momentous event that it is no surprise to see it ranked #1 on the list - it gets referenced CONSTANTLY. They even adapted it into the recent Amazing Spider-Man film franchise.
From the "Snap" to Spidey trying to convince someone (himself? God?) that Gwen isn't actually dead, that's still a painful thing to read. But an extremely memorable thing to read.
Okay, that's the list! Happy Seventy-Fifth Anniversary, Marvel! Here's to seventy-five more years of more memorable moments!