Captain america's death was a stupid gimmick that was only saved from being yet another event driven piece of drivel by the way it was brilliantly written and followed up by Ed Brubaker, who turned something unbelievably idiotic on every level into something really good. But then we came to Captain Americas return... which just made no sense. It was obviously an event driven gimmick just to bring him back and any common sense had been thrown out along with the several years worth of quality scripting.
As far as the best character deaths? There have been a few good ones. Even if they were later invalidated. And in looking at it, the best ones seem to be the characters that no one expects to come back. They are just far enough out of the A list to not have the instant reset button, while close enough to the top to have some lasting impact. A few whose deaths impacted stories for years in a mainly good way.
The best of these was probably Barry Allen, the Flash. Barry Allen as the Flash was one of those characters that you would love to live next to. The nicest guy on the planet. A true boy scout. But because of this his books were kind of missing something. But his heroic sacrifice in the original Crisis was stunning at the time. And even more was the lasting impact. Barry Allen became a far more interesting character dead in comics then he ever was alive. The impact on his sidekick Wally West, the impact on the Justice League. just some really good stuff came out of it. For decades. Then they went and messed it all up by bringing him back. Bah!
Also high on the list for good hero death would be Captain Marvel. (The Marvel one, Mar-Vel of the Kree). He died in a special graphic novel. The first of its kind. No dramatic build up in a series. He died of cancer, in bed. It was one of those stories that seems a little cliched and cheesy today, but was quite shocking at the time. Once again the underlying take away was that there would be no reset button, and the death would have some long term impact, no matter how minor. After seeing years of comic character deaths it's still one that stands out.
The death of Guardian in issue 12 of the original Alpha Flight series. Still one of the most shocking pages ever in a comic. Possibly the one single moment when John Byrne surpassed the death of Pheonix. And once again it had a lasting impact for years. Heck Byrne himself even dangled the reset button in a followup story arc, only to slam the readers again with yet another mindf__k 12 issues later. Once again that death had long term effect, at least until some editorial decision said they needed some publicity decades later.
I'm not sure if this counts as a superhero death, but the death of Morpheus in the Sandman stands as one of those truly compelling moments. You knew it was coming. The entire series built to it. It was Destiny, it was inevitable. And it was still powerful.
Oh and speaking of Mindf__k's, that particular characters death over in Adam Warrens oddly charming borderline sftcore title Empowered definately comes down on the side of good powerful story driven character death.