Source: hero.wikia.com

9. Captain Marvel

(Captain Marvel, Marvel Comics)

The Death of Captain Marvel is a super downer of a graphic novel, mostly because it's about a superhero dying of cancer and that’s inherently pretty depressing.

It was revealed that after being exposed to Compound 13 in his fight with Nitro, Mar-Vell contracted terminal lung cancer. Fortunately, his powerful nega-bands prevented Mar-Vell from immediately dying, but they also prevented any treatment from being administered.

He left for Titan to spend his last days in peace. And he did, surrounded by his former partner Rick Jones, his lover, and The Avengers, his closest friends. The Skrulls even honored him as their greatest enemy. In his final moments, Mar-Vell was greeted by his greatest foe, Thanos, and Mistress Death. They welcomed him to the afterlife, not as an enemy, but as a rival.

And one truly deserving of a good death. Mar-Vell sorta came back a couple of times, once during Civil War as a Skrull, and once as a resurrected soldier using the M'Kraan Crystal. Mar-Vell died both times, and his soul returned to peace.