From the fan perspective, it would be SUPERMAN vs THE AMAZING SPIDER-MAN (the first one). I don't think anything will -- or can -- ever be that cool again!

Amen to that.
It was published as a oversized special in sweden and I still own it - it's bloody awesome.

A close second is a Kirby/Lee season with FF - in oversized black and white where I could study the artwork in detail - and the first time FF went to Latveria as agents for Shield after returning from a battle with the inhumans, all published by Red Clown in sweden.

Closely followed by the Dark Phoenix saga in X-men, which escalated until it became *the* defining X-men story, after a slow, cool build up.

Then, the (again) oversized collected Star Wars, depicting the first movies events, with a young Howard Chaykin as the artist.
I read it before I saw the movie - and it didn't spoil it for me.

And finally a swedish oversized special again, depicting Batmans first encounter with Ra's Al Ghoul, showing off Neal Adams in all his glory.

Damn, that was good stuff....

Funny you should mention that series because I think it was/is in no small way the reason why there ARE any fans in the first place!  I am not quite old enough to say this with authority, but get the impression that before this series, there were people (usually kids) who read comics...AFTER this, there were comic fans...

...not that I am blaming Stan and Jack for anything you understand...

I'd like to split my answer , The coolest comicbook moment for me was during the Infinity Gauntlet Mini-series. All the gathered hero's were out of action except for Captain America( Silver Surfer & Adam Warlock hadn't entered the fray yet)Thanos stood victorious and Captain America slugs him.And makes a statement about him still standing. Now Cap had to know he was but a speck on the wall in the cosmic sceme of things yet still the Captain stood toe to toe before going down.

The coolest momemt for me was when i was ten years old my extended family moved to Long Island While visiting i discovered a comic store with a sea of back issues.That where i discovered John Byrne, Jack Kirby, Romita sr, The Buscema's, Gil Kane,Dave Cockrum,Neal Adams ,Art Adams, Frank Robbins and Mike Ploog.The found The Invaders,Moon Knight,Ghost Rider, Werewolf By Night Son of Satan,Tomb of Dracula and Adam Warlock. I was in heaven .I was ten years old and 20 dollars in my pocket and a 60 percent off on all back issues i cleaned up Like 40 comics owe to go back to those days

The coolest moment: Anytime I witnessed Superboy meet up with Superman in any fashion.

Coolest moments for me-

Dr. Doom beating the Beyonder-my first experience with the coolness of Doom-and several other moments of Secret wars-and the whole coolness of seeing all of these heroes with Spider-man and the Hulk and discovering a Universe.

Spider-man beating Venom-I was 10 or 11, and man, I could not figure out how Spidey would be able to beat someone tougher, stronger, nastier, and crazy, who could avoid his Spider-sense.  I just don't think I can ever get that feeling back again-will Spider-man get out of this or not?  I was afraid for him, and thrilled when he got the sonic gun from the FF and used it, and discovered that the costume was permanently attached to Venom. Wow

Anthony - thanks for sharing that story of when you were a kid.  It brought back memories of when I was a kid and first discovered Captain America on the newsstand shelf (remember when comics were sold at the local newsstand?)

You all so far have some great input here...

Hard for me to say.  I've mainly collected Cap and Batman all my life and I'm not as varied as many of you in your comic experiences.

I think the story that has stuck with me (not kissing hiney here either) was the Baron Blood story in Cap during JB's run.  The way that JB and Joe Rubinstein (colorist needs a nod here too) illustrated Cap as the sun was going down, and he knew what he had to do to eliminate Baron Blood as a threat - he chopped off his head with his shield.  After, Cap walked away, and was SAD at what he had to do, to take a life to save everyone else.

I could go on and on about it, but for me, this story really set in me that Cap was a HERO first, and hooked me forever on Cap, JB, and comics.

Jon Godson wrote: Oh, my god, this is why John Byrne pisses me off sometimes. The coolest comic moment ever was the Dark Phoenix Saga, which was what comic books had been leading up to for fifty years. .

He pisses you off because his opinion differs from yours?

Weird...

I'm with the camp on Superman VS Spider-Man.  My favorite of all time.  Carlos Pacheco has shown a strong love for it in many of his books with little references thrown into the background.  Avengers Forever has a tiny shot of Luthor astride Doctor Octopus in an alternate diminsion sequence and I can't remember where it was, but there was a young photographer wearing a familiar black sweater in a panel or two of a comic he drew as well.  I know there are others that escape me at the moment. 

My favorite line from Superman VS Spider-Man:

"I think MJ was right!  Maybe I DID have nails for breakfast!"

Jon Godson wrote: No, he pisses me off because his humility won't let him see what an impact the Dark Phoenix Saga had on the comics industry.

How do you get THAT from his answer above?

SO hard to narrow it down, given that I have been reading comics for almost 35 years, but I'll go with the first three that came to mind:

01.  Banshee and Marvel Girl learn Wolverine's claws are part of him.

02.  Two words: Galactus Falls!

03.  The Amazing Screw-On Head