Ah, comic books... where dead doesn't really mean dead, just a wait. There are some characters who have remained dead, but it's so often for characters to come back to life somehow that this is called the Comic Book Death. One of the most common ways of doing this is to retcon it, by saying they somehow survived and got better when that was never intended.
But what are the longest waits between death and revival?
I think the record for A-list villains is Norman Osborn / Green Goblin. He died in The Amazing Spider-Man #122 in July 1973, and didn't come back until Peter Parker: Spider-Man #75 in October 1996 where that death was retconned by a healing factor staving it off. That's 23 real world years of no Norman Osborn in the MU, but he did leave behind a major legacy of successors and impersonators, and him killing Gwen Stacy forever changed Spider-Man's stories. Funny thing is, he's been making up for lost time, as despite taking a massive break, he's still the Spider-Man villain with the most appearances in comics (him becoming a General MU villain in the '00s likely had a big part to do with it).
Anyways, what other notable examples are there?