I went with my favorite storyline. Of the ones listed, "Born Again" is my favorite.

As much as I was impacted the first time, and do when I read it again, by Born Again, I have to go with The Death of Captain Marvel. It was real and possible and one of the things that in actual reality has and continues to plague mankind with still not cure in sight. Marvel died as no hero (that I have ever read yet) had. Even to the point that Thanos was actually angered that Marvel died in such a demeaning manner, to the hero he was and wanted to honor him as a warrior, not dying weak and helpless in a bed. Very powerful stuff.

Don't forget, Sue Dibney (nee Disney) was retroactively raped, as well as murdered.

Other:

Dark Avenger of the night vigilante adopts child and puts him in brightly colored costume and in harms way...Detective #38, one of the biggest changes to a character concept ever and one that radically changed the nature and direction of the book.

-M

Jonah Hex of the Old West becomes Hex, whatever the hell that was.

    

This was another radical transformation:

 

I think 'biggest' and 'shocking' are two different things.

I think Tony Stark becoming a homeless alcholic was probably the BIGGEST change, but it wasn't shocking... it was lead up to for some time. For shocking, I'd go with Barbara Gordon.

There are a lot of real strong candidates here.

I'm gonna throw my vote on Hal Jordan becoming Parallax.

Superheroes have died, become crippled, become drunks, retired, whatever.

DC took one of their headline characters from the dawn of the Silver Age, a founding member of the Justice League, a character who predated the freakin' Fantastic Four, and they turned him into a supervillain.

Did Marvel do it first, with Jean Grey? Yes. And DC even, eventually, copied the same cop-out retcon to undo the damage. But Jean Grey was not Hal Jordan. Jean Grey was one member, and IMO a lesser member at that, of a team. Hal Jordan was the lynch pin of an entire mythology, an entire corner of the DCU had its roots in Hal Jordan.

And he became a supervillain the same way so many other supervillains did: Trauma overwhelmed him so completely that he lost his goddamned mind. And he didn't just get nuts, and eat a star that happened to be host to billions of lives. No, he renounced the Green Lantern Corps, flew to Oa and killed his own supporting cast, destroyed the Central Power Battery and declared himself a new supervillain. He went so hard into the world of supervillainy that the Green Lantern title was taken over by a new kid who was the only Green Lantern left in the universe, to whom Hal Jordan was the symbol of everything that could go right and wrong.

And then, after two years of this, he finally sought a measure of redemption by sacrificing himself to save the Earth. After that he was completely absent, dead and buried, for three years.

Like I said, it had been done before with Dark Phoenix. But it hadn't been done to the same extent, with the same dedication. It hadn't been done to a star that had shone that bright, and they had never taken someone down to those depths. Killing Superman? Big deal, we knew he would come back. Breaking Batman? Please. Killing the Flash? Wally West was waiting in the wings to take up the mantle. But this? This had major balls.

If you look at the ramifications of this storyline, they're massive. Because this story is what ultimately, ten years later, leads to Geoff Johns getting on the Lantern books. And once he's on the Lantern books, that corner the DCU explodes in a huge way. The mythos got expanded exponentially, brilliant work. And from there, that led to everything Johns has put his paws on since.

Huge storyline, huge shock, huge consequences. That's my pick.