Green Lantern Corps and Silver Surfer
If there is one death at DC I would like to undo, it's that of Katma Tui.  I always liked her character and was glad when she became a romantic interest for John Stewart, thinking she would get more to do.  Unfortunately that made her ripe for a sacrifice to motivate John and her fate was sealed.  It hurt even more when the grisly moment of her death at the hands of Star Sapphire was illustrated by legendary Green Lantern artist Gil Kane, making it seem more "real" to me. I thought that with her appearances on the animated Justice League Unlimited, she would make a comeback but alas I have not seen her return.