I have three and two of them are recent events.

Judge Dredd: Armageddon Wars.  I never read any Judge Dredd stories as a kid because I didn't have access to them. A few years ago I started buying the black and white JD reprints on a whim and really liked them.  After reading the multi-part Armageddon Wars story I think I said 'wow' outloud to myself.

That experience was very similar to how I felt after reading the first volume of JB Compleat Next Men.  I was blown away.  This was about a year before the Dredd story.  I recall telling a friend that it wasnt a comic, it was a sci-fi novel.

My third was JB's FF story where the Thing 'fell for a day.'  My friend and I talked about that experience for a long time.  We wondered if you'd get hungry, sleep, etc., since, as we saw it, fear and panic would eventually wear off. I think this was my first 'imaginary story*' so I was buying everything hook, line, and sinker as  comic book reality.  "The FF is being killed off!  What?!"

*maybe imaginary moment is a better term?

First Post-Crisis encounter between Superman and Joker, written and drawn by our host, of course!

It was years after that title that I learnt Superman and Joker had had some pre-Crisis appearances, but for me, that was the FIRST Superman/Joker encounter. Wow. Of course, I knew they existed in the same fictional universe, but prior to 1987, I had no idea they'd ever meet. And our host gave it to us!

Doc Samson knocking out the Hulk, again from our host, was another wow! moment.

And discovering the Superman/Spider-Man crossover was a wow! moment, too. This was BEFORE the internet. I had no idea it existed until I saw it in the bookshop next to the railway station. I asked the guy next to me to pinch me to make sure I wasn't dreaming (okay, I made that last sentence up).

Right up there is JB's FANTASTIC FOUR ANNUAL #17.  That started as a "Wow." and ended up a "Wow!"

A 3 part Defenders story (which was huge for me at that time) that built up to a heart wrenching ending.

Long after I had stopped buying comics I would let this story drift back into my head every so often.

I have recently reacquired it.

The JSA run in All-Star Comics leading up to the death of the Earth-2 Batman was huge for me. I eagerly followed every summer crossover and very much enjoyed following the team wherever they went. Being older, they were to be accorded more respect, I felt as a kid, and I did so. Crisis threw everything into a non-stop randomizer where anything that happened could be made to un-happen, then re-happen only maybe uglier this time, or good guys could be cynically rewritten as having been scumbuckets all along... Ah, but before everything could be made neverwas or simply foul, that JSA run was a goldmine for me. The Psycho-Pirate story which led to a trigger-happy guy on the GCPD opening fire on Power Girl... whoof. And the reaction everyone has later when Superman, the original, comes through the wall... Damn it, these characters commanded respect. Truly, there's been nothing like that run since. What others felt was present in Waid & Ross's "Kingdom Come," I found here long before then.

X-Men by Claremont & Byrne- I came in when the team was held captive in Magneto's volcano base, stuck around for the Savage Land story, vanished for awhile and came back for the upshot of the Dark Phoenix Saga and basically stayed for years afterwards based on the memories of the power in those early issues. Man, those characters really hit home. When Hank and Jean returned to the mansion with only the ghosts of the others on the cover of #114, I could feel that ache and grief... 

Showcase 94-96: The New Doom Patrol- Another storyline connected with inconsolable loss and grief and the aftermath thereof, and again, these people will pick themselves up and go forward, no matter the cost. Before the first issue, I bought comics occasionally. A few this month. A few months without any. After it, I went hardcore junkie, buying as many as my parents' income would allow. Maybe some other story would have triggered the same reaction later, but this is the story that turned me into a full-time comics reader. To this day, I still love that Rog-2000 inspired version of Robotman. 

Daredevil #192: What would it take to make a good man go wrong? How do you put him back together once he's decided to make a mistake, knowing it's a mistake? One of the very few comics that's ever made me cry. An oft-overlooked issue by Alan Brennert. One of the most honest and genuine issues looking at these characters I've ever read. 

Swamp Thing #32: "Pog" Hit me like a falling stack of bricks. Loved it. Again, one of the very few to bring on tears. So much fun to read throughout, and then... Then... See also Moore's "In Pictopia" from Anything Goes #2.

Looking back... I miss giving a damn about characters and story. It'd be nice to find a book again that made me think the creators did.

Doug, I remember that story very well! I only ever read one-third of it as a kid, having traded comics with a friend to get the issue. I tried catching up with it years later, as an adult, and found out it was STILL issues and issues later before they told us how Moon Knight got out of that death-trap! :-)

DARK PHOENIX, obviously (the story that made me start hunting out American comics as opposed to waiting for the UK reprints - I just couldn't wait for the next part (of course, at this point, I still had not figured out that I was still three months behind and they were coming over on a boat. I also couldn't figure out why some issues like 137!!!! wouldn't turn up!!!))

DARK KNIGHT and WATCHMEN (among the stories that made me start to trek to Leeds every week and buy American comics on a weekly basis as opposed to waiting for the three month delay for them to arrive in a newsagent)

BORN AGAIN (the other story that made me do this)

MARVELS (painted artwork)