Tony DiMera. In stint one, you had a good guy who warred with his mother's ex-husband, Stefano. In stint two, Tony starts out as a pretty noble guy, but one that ultimately will protect his father, Stefano. After being betrayed while blind, he gradually becomes unhinged and commits suicide and makes it look like John's the killer. Since I had not been watching 10 years earlier, stint two was pretty good to me. Then the stints start running together. Tony's not dead. Tony gets more evil. Tony sets up serial killings to look like they were done by Marlena and fakes his own death with a tiger attack, then the serial killings turned out to all be fake. Tony's deep in the character hole.
Then, in 2007, Anna gets wind that all this was Andre, who had originally been killed off in 1984 but isn't dead. She and a couple of others go to this tropical island and there's Tony alive and well and has no knowledge of anything going in Salem since 1985.
So you have character destruction, a new set of writers realizing how awful it had gotten but instead put the character in a bigger hole trying to fix it.