In The Darkness Saga, when Darkseid is finally revealed, standing on the ruins of Daxam.
Ambush Bug in the Superman museum in the 30th century: “One more step and I’ll kill the cake!”
'Mazing Man saving the child from the truck in his first issue.
Death of Supergirl.
My #1 - Clark revealing his super identity to Lois. I mean, come on. It was 60 years in the waiting!
The Hostess Page from Seanbaby.com. This is page is probably work safe, although I know that can not be said for all of the Seanbaby pages. Also, since my browser blocks pop-ups, I can’t know if some unsuitable-for-work pop-ups are lurking.
Also from Whatever Happened to the Man of Tomorrow, the splash page looking down on Superman as he cries in the Fortress of Solitude following his visit from the Legion (with Supergirl in tow). I find it much more heartbreaking than even Superman #75.
Also, from the same story, the death of Krypto. Sniff.
From The Golden Age -
Alan Scott: “Villain. You are history.”
Tons from Watchmen, but my favorite is probably (paraphrased), “End? Nothing ends, Adrian. Nothing ever ends.”
Mongul’s killer line from SUPERMAN ANNUAL #11: “Which of you would it be polite to kill first?” I hope it stays in the animated JL adaptation they’re doing.
Nah, “The Boy Who Collects Spider-Man” is about Spidey meeting a kid who collects newspaper clippings and other “souvenirs” from Spidey’s various escapades. Accompanied by a newspaper column by Ben Urich.
Definitely worth hunting down if you haven’t read it; I think it’s been collected in some of the “Best of Spider-Man” collections Marvel puts out.
The panel in *Superman Annual * #11 where Superman gets pissed.
I always hated Supergirl. Until the scene in *Crisis on Infinite Earths * where she died heroicly.
In one of the early issues of Sandman, Morpheus is in Hell. Every demon in Hell is standing between him and the gate. And he talks his way out.
*X-Men * (#135? #137? I forget) The death of the Phoenix.
In Watchmen, when the villain explains his Master Plan.
“When were you planning to do it?”
" ‘Do it?’ Dan, I’m not some Republic Serial villain. Do you seriously think I would explain my masterstroke if you stood the slightest chance of affecting the outcome? I did it 35 minutes ago."
… is indeed the greatest moment in comic book history - and I’m not even a fan of superheroes.
A special shout-out to the poster (whose post I can not know find) for pointing out the “Death’s younger brother”. That is indeed a truly great moment.
A few others:
Valerie’s letter, and “Give me a Viking funeral” from V for Vendetta.
The tidal wave crashing down on New York, Marvels.
“We do deadlines”, Transmetropolitan.
The Saint of Killers sitting slumped on the throne of God, Preacher.
Less well-known, but one I’ve always been very fond of (paraphrased):
“You are in a cave, in the middle of a rainforest, below the good clean earth. You never had a chance.” Swamp Thing.
In the “Emerald Twilight” storyline from Green Lantern (a substory of the “Death of Superman” saga) when an anguished Hal Jordan (having killed or maimed numerous fellow Lanterns for their power rings) realizes he doesn’t have the will power to continue the illusion of his destroyed Coast City.
The Avengers issue where Capt. America finds all of his personal things have been destroyed by Mr. Hyde, and there is a tear as he picks up the shreds of the only photo he has of his mother.
I’m gonna nominate something from Sluggy Freelance. Yeah, I know the series has it’s problems lately, and it’s not exactly superheroic, and it’s primarily a web comic, but still. You really have to read all the preceding strips for the full effect (or at least read The Love Potion arc), but here’s a brief summary:
Oaisis is possibly a robot or cyborg, trained as a gymnast and assassin, and is obsessed with Torg to the point of trying to kill anyone else who might possibly be attracted to him. Torg and Zoe have a longstanding “are we friends or aren’t we” thing going on, and Oaisis has attacked Zoe because of this. In the following strip, Torg has told Oaisis that he’ll marry her to try to keep her away from Zoe.
I knew when I saw this thread that my two favorties would have already been mentioned, and it makes me proud. My mouth literally dropped open when Veidt let loose that line. That line was the dividing line between the old and new ages.
Also, the very last issue of Grant Morrison’s “Animal Man,” where he steps into the comic and spends the issue telling you about how he had a sick cat, and he did everything to take care of it and nothng worked. And then he goes on and on about how he had an imaginary friend when he was a child, but he feels that as he’s grown older, all of the magic in the world has died. And he signals with his flashlight to this friend, but he knows that none of it’s real because magic is dead and etc. etc…
And he walks away, and the very last panel is the other light signaling back.
I bawled in my car reading that.
The first issue of BLACK PANTHER (Marvel Knights line), when the ultra-smooth T’Challa, flanked by the fine-ass Milaje Dora, uses Wakandan tech to stun a carful of thugs, then grab one by the roots of his hair and run up the side of a building. “We shall have an understanding. Speak of it to no man!” says the Panther, echoed the Phantom pretty well. Everett Ross’ take was funny, “If I could have sold tickets, I could retire.” My only regret is that this kick-ass take of T’Challa was never really followed up on during the next 59-issue run. OTHER FAVORITE MOMENT: When the X-Men’s Storm dropped by for revelations and a kiss, prompting T’Challa to FINALLY admit he is sometimes afraid and doesn’t know what to do.
In the DAREDEVIL: BORN AGAIN story, there are lotsa good moments. My all time favorite is watching the Kingpin squirm with glee as his thug describes Matt’s vicious attack on a gang of subway theives as proof of how Matt Murdock is slowing cracking up. “How I envy you, you worm. You have witnessed the death of a noble man. The pleasures of his coming attack are nothing compared to this,” croons Kingpin. “You think he’s coming here?” “I know it.”
Watchmen
The entire page where Rorschach and Nite Owl are remniscing about the memories they had before the Keene Act outlawed any super-hero not backed by the Government (which was only one… person); the Nite Owl had returned to a normal life, while Rorschach was operating as an illegal vigilante. Nite Owl asked where those wonderful times have gone; those years when they protected the innocent; their past lives spent fighting for Justice. Rorschach merely replied:
“You quit.”
And then the subsequent frame of Nite Owl sitting dejected beside the open closet which housed his old costume, looking painfully human in his suit and tie.
And the very beginning entrance in Rorschach’s journal:
"Dog carcass in alley this morning, tire thread on burst stomach. This city is afraid of me. I have seen its true face.
The streets are extended gutters and the gutters are full of blood and when the drains finally scab over, all the vermin will drown.
The accumulated filth of all their sex and murder will foam up about their waists and all the whores and politicians will look up and shout “Save us!”…
… and I’ll look down, and whisper “No.”
In Preacher: Gone to Texas, I’m still utterly fascinated when Cassidy intends to destroy the Saint of Killers by ramming his truck into him, and instead ends up wrapping his vehicle around the Saint of Killers (who didn’t even flinch). He ended up lying on the ground, with shards of glass embedded in his face and other injuries that would have killed a mortal. Smiled as he lay on the ground,
“Cheer up preacher man… you’re saved.”
And in the Invisibles
“Your head’s like mine, like all our heads; big enough to contain every God and Devil there ever was. Big enough to hold the weight of oceans and the turning stars. Whole Universes fit in there! But what do we choose to keep in this miraculous cabinet? Little broken things, sad trinkets that we play with over and over. The world turns our key and we play the same tune again and again and we think that tune’s all we are.”
My favorite moments come from Swamp Thing: American Gothic (the line already mentioned – “AND YOU SHOULD NOT HAVE COME HERE…”, where the evil bad guys are trying to overpower Swampie with magic, but he is the Earth elemental, and much more powerful than they are) and from Daredevil: Born Again, after the Kingpin/Daredevil fight.
[spoiler]Kingpin puts Matt’s unconscious form in a taxi, plants a body, and traps Matt in the taxi, which gets sent to the bottom of the East River. Kingpin gloats when receiving the report from a lackey, except…
There is no body.
There is no body.
There is no body.[/spoiler]
I also like a moment from the same Daredevil series where Matt uses a stomped icicle to stop Karen from shooting up, then hugs her. I wish Kevin Smith hadn’t killed her off, but that’s comics.
All the really good things have been mentioned, so I’ll do some piddly ass ones from my limited comic book geekdom.
In the X-Men, when Magneto ripped out Wolverine’s adamantium. All those years of just tossing the little runt around, it was about time Mags took it to the hole. Watching that happen was one of the most shocking and terrifying bits of comic book history for me, because Wolverine was my favorite character at the time. Unfortunately, Marvel really dropped the ball with the aftermath, but at that moment…damn!
Ab-so-f’ing-lutely.
The Authority has a lot of great moments, too. Superheroes who are very willing to kill for what they think is right, in the hands of a great writer, create some amazingly good moments.
“I’m Jack Hawksmoor. I’m the god of cities, and I just told Paris not to take any more of your shit.”
While I wouldn’t pretend to know enough to identify any one moment as the greatest in comic history, these are the moments that stand out for me.
Amazing Spider-Man #38: “The Conversation”. Aunt May figures out the Peter is Spider-Man and confronts him. One of the best superhero comics ever consists entirely of two people talking.
In the late 80’s, there was a Christmas special. One of the stories involved Deadman, who uses a rich man to donate some money, then jumps into a young woman skating in Central Park. He stays too long, effectively ruining Christmas for her and her family when he leaves and she has no memory of the previous few hours. He complains aloud about all of the good he does without anyone ever knowing about it.
A young woman chastises him, asking him why “we” do it, making it apparent she’s also in the guardian angel biz. They talk, and he begins to feel better. As she walks away, she tells him her name is
Kara. This is a nice, sublte reminder that just prior to that she had saved the entire universe, but nobody would ever know because in the process she was erased from existence.
•The bit of scene following Supermans confrontation with Magog in Kingdom Come. (“You must be proud…” and on, and soforth.)
•The death of Jason Todd.
•Any of the times that The Joker has died, in AUs, elseworlds, and whatnot.
-Add to that, Batman’s reaction after killing the Joker (after he killed Robin and Batgirl) in JLA: The Nail. Yowch.
•Maybe the time that Peter Parker and Mary-Jane’s daughter was stillborn. 
(Though, as I remember, there was some sort of hint that she wasn’t really dead, and she was really kidnapped by the Green Goblin or something, but they never really followed up on it.)
From Kingdom Come: “And the Batman…the Batman has his city under control.”
That’s terrifying.
And of course the bit in KC where
Batman and Superman are speaking in the Batcave and Superman does that VANISH at superspeed thing while Batman is speaking to him then Batman says, “So that’s what that’s like.” That’s funny.
Or the final joke in The Killing Joke. Forcing Batman to face the inherent similarities between himself and the Joker…