Many, but I will say Bucky Barnes. When Stan Lee decided to retcon Bucky's fate in The Avengers #4, instead of getting rid of teen age sidekicks (which he has repeatedly said he never liked), he fueled the opposite reaction: turning Bucky into (arguably) the second most important teen sidekick in comics history, right behind Robin/Dick Grayson. More to the point, he also gave the revived Captain America one of the greatest, most effective weights of tragedy of any comic character. The entire re-introduction of Bucky--from endless flashback stories, character recollections, to impostors, androids, robots, replacements and time travel, Bucky was that one, interesting, yet tragic figure who stood as the ultimate example of a grave, heroic sacrifice that could not be wiped away.

It shaped Captain America's life and decisions for decades, while providing that spectre that the Marvel Universe--when killing a non-powered teenager--was not a safe place at all. You rarely experienced that feeling from a super-powered character dying, as they were--at least--some equal footing to or not as vulnerable as a superpowered and/or enhanced enemy. Bucky was just a well-trained young man who battled through a world war and beings far beyond his human limits, but he never turned away from it, so with his death, Marvel felt like a place of real consequences. That was all wiped away with the Winter Soldier plot, which was just another tired spy trope (the old, brainwashed / "I thought you died, but now you're an enemy!") on comic-book steroids, nearly rendering all of the considerable dramatic capital built for Captain America as irrelevant. 

One can say, "well, those old stories still work because he actually thought Bucky was dead", but the Winter Soldier idea just boiled all that down to, "well....he was wrong", erasing the once dark legacy Cap had to live with. That Brubaker story just makes all comic superhero death stink as the next "shock" gimmick. 

Death is powerful. I would agree that I think Jean Grey opened the door for it. Elektra's death, for me, was powerful but her revival was also shrouded in mystery and still worked for me.

I'll agree - Jean Grey, not least because the way the resurrected her was such a dreadful story.

Up until that time, death was moderately real in Marvel comics - if you didn't actually see the character dismembered "on camera", you knew they were fair game to come back, but once Jean came back, actual death became absolutely meaningless, completely vitiating the impact of any character's death, because you knew they'd be back, generally within the next year or so. You could sort of argue a case that a character called Phoenix should be able to come back, but it opened the gates for everyone to come back, without even sensible explanations for how them came back. The characters even talk about death being temporary in the books themselves!

The really stupid thing about bringing Jean Grey back was that she wasn't even a particuarly good character, and hasn't been especially well used since.

Also agree that bringing Boring Barry back as the Flash was a terrible decision - the relaunch of the Flash with a muich slower Wally, after CoIE, made a really stupidly over-powerful character interesting again, not that that lasted for long.

You'd think they just had Phoenix/Jean Grey reassemble themselves like that Klaw sonic villain guy was often enough doing. I don't get how Jean not having been Phoenix after all made anything cheapened? It was her copied humanity that made the Phoenix entity at all human, and to have Jean Grey not corrupted to pure evil by all that Hellfire club and Mastermind decadence junk was a victory to me. I'm totally glad she was salvaged so logically from the mess Shooter and Claremont imposed, she deserved a better fate. Phoenix destroyed a solar system and a ship full of people, they'd wanted her to become a villain, but I'd rather go with the Phoenix had to die line, but Jean Grey deserved to live to me... of course then some fool had to put them together again, and yatta-yatta-yatta. Argh.

"Phoenix" is the most obvious answer since her death was such a critically acclaimed and fan-approved story, and her resurrection undertaken for dubious business reasons that undermined the direction of the story. But it was just pointed out in the X-Men thread that X-Men had already resurrected Professor X, who was dead from issues #42-65 and then brought back with a "No, I was just pretending to be dead so I could get some work done. Hope you didn't miss me too much..." So I can see how Shooter and the X-Factor folks felt they were actually standing on precedent, not breaking a sacrosanct tradition.

Death of Superman is the exception to the rule. If I recall correctly, when DC first announced the event, it was a slow news day which is why some of the larger news outlets started carrying the publicity. Speculators became immediately invested while casual fans who may have known of Superman only through TV and movies suddenly were shocked that DC would kill an American icon thus swelling the popularity. 

As for the event itself, to the best of my knowledge, there was more involved than it perhaps being simply a sales gimmick. Group editor Mike Carlin and his creative teams had apparently wanted to marry Superman at the time but this effort was put on hold when it was learned that Superman was going to get married on the Lois & Clark: New Adventures of Superman TV show. In an effort to find a new direction at their yearly creative summit, it was decided to pursue the notion that had been offered before jokingly (I think by Stern) of 'Let's kill him'.

The story line lasted for nearly an entire year and ran through four concurrent titles, hardly a speedy return. Sure, comic fans knew that he wasn't going to stay dead so that only added to the growing mystery surrounding the appearance of the four new Supermen. Besides creating new, lasting characters as both heroes and villains, the creative team went the extra mile in showing the effect of Superman's death upon the DC hero community, his supporting cast at the time, and the DCU in general. So it wasn't just some mere sales stunt, it was an engaging epic that lead to new layers and themes and characters for the entire Superman mythos. Quite simply, it's a classic.

As for Jean Grey, yes, sales were part of the formula involved in bringing her back, in re-uniting the original X-Men to entice and excite new and old fans alike. However, I have little issue for how they brought her back, based upon Busiek's idea, Stern & Bryne crafted an amazing rebirth that did not alter the impact of the DP saga. The original saga still holds up on it' drama and emotion while this rebirth simply added a new layer of mystery and drama to the event. It expanded themes for the Phoenix Force and it may have also wiped away the supposed crimes of Jean herself. For better or worse, the consequences of this rebirth are still being felt today.

No, in answer to the OP, I have to agree with tarkintino, it's Bucky Barnes. For me, the main difference here is that Jean Grey never suffered from the stigma of 'needing to stay dead'. Jean was 'dead' from '80-'86 and while Claremont showed the effects of her passing on the team and her loved ones, I never got the feeling that she was required to stay dead in order for the characters to be relevant. (Yes, let's be slightly pedantic, it was in her code name for goodness sakes).

Unlike with Bucky, who, as tarkintino mentioned, was given tragic relevance for Cap since his revival in our modern era. The stigma of that tragedy has haunted Steve for decades and provided an extra dramatic underpinning for a man out of his time. I've always heard that the only two characters who can never return from the dead are Bucky and Uncle Ben.

That's what makes Brubaker's accomplishment so amazing and frustrating. Again, in these instances, the creativity involved in how these rebirths are handled is important, Brubaker fashioned a compelling and entertaining story that undid a hero's tragic death and reforged a comic bond of friendship through hardship and triumph. Fans loved it and the Winter Soldier has since become quite prominent (imagine what the MCU films would be without him).

By the same token though, Brubaker showed that with such creativity, no death can ever be viewed as permanent even the ones thought to be set in stone. Jor-El is back, Thomas Wayne returned as the Batman of Flashpoint and through events in Rebirth, had a profound effect on Bruce's choice to remain Batman. Heck, even Uncle Ben has come back as once seen in Slott's Spider-Universe event where in an alternate world, Peter learned that it was Ben who was bitten by the spider and not him.

The only death seemingly immune to this effect is Mar-Vell. I think it's because of the close association of Starlin with the character as well as how he died, not from some super-villain's mad scheme but the very real threat of cancer.

I know you are a big Captain America fan as I am from your great posts in my review thread, but I will respectfully disagree with you here.  The fact that Brubaker wrote a story, contrived or tired as you feel it may be (which I did not) to explain how Bucky, long thought dead but somehow alive, does not erase Cap's "dark legacy" or render the dramatic capital for Cap as irrelevant.  Let me use a close, but not perfect, analogy to explain:

I'm dating a girl in college, both of us hopelessly happy with each other and considering marriage.  I buy the ring, she says "yes", and we're making plans for our future.  One day, out of the blue, she returns the ring and says she can't marry me, then drops out of school and despite my best attempts, I am unable to contact her; she's become a ghost.  I'm devastated, wondering where it went wrong, if it was my fault, and what I could have done to make her stay, and I become increasingly bitter about relationships and love.  I spend the next 10 years of my life frequenting strip clubs and picking up women of questionable morals in bars for one-night stands, figuring the only way to keep myself safe and not get hurt again is to avoid all meaningful relationships and just focus on my basest needs.  My life is in a tailspin of alcohol, drugs, and pointless sex, affecting every thing and every one around me, including my job and my relationships with friends and family.

Then, out of the blue, I'm traveling for work and I run into my college love in another town.  She avoids me at first, but I manage to get a conversation out of her, during which I learn that she had gotten pregnant but miscarried our child while we were in school and, unable to face me with the news as she knew I wanted children with her more than anything, she ran out in shame and pain.  Knowing I could find her at her parents' home, she moved in with a cousin and finished school elsewhere, hoping to forget the past because of how she felt she had let me down.  We talk further and realize that we never stopped loving each other, and from there, we start to rebuild the relationship we once had.

Now, does the fact that I thought her gone ("dead" to me, to carry the initial conversation about Bucky into here) erase the "dark legacy" of my decade without her?  Does the damage I did, both to myself and those around me, suddenly become moot because we one day reconnect, and am I ever able to really escape the torture I put myself through for those 10 years?  Absolutely not, because every moment of our lives shapes the next moment, both the good and the bad.  Even if this girl and I managed to rekindle our relationship, even to the point of considering marriage again, there would always be part of me wondering if she was going to run out again or if my past could come back to haunt us. 

To me, the old stories still work, because of the very fact that Cap acted on his assumption, wrong as it was, that Bucky was dead.  He was haunted by the memory of his "dead" friend, tortured by Bucky's image by numerous foes, and even tried to replace his companion with others (Rick Jones, Jack Monroe, Lemar Hoskins, etc.), all because of the pain he felt because he BELIEVED his friend to be dead, even though he had no proof.  The fact that Brubaker revived Bucky doesn't erase or diminish the legacy but adds to it, making it richer, because Cap then had to process the fact his friend had been turned and he was going to do everything he could to save him as he could not the first time.