I pick Abigail Johanna Deveraux DiMera DiMera Hernandez DiMera DiMera.
She never died! She faked her own death and framed poor innocent Clyde for her murder AND her drug-running operation, all in the guise of a mild-mannered reporter and DiMera housewife. She was so powerful and had such a strong organization that even Clyde didn't know she was his boss.
She's also behind the Cat scheme, caused their car crash, and tried to program Cat to replace her.
Her return shakes Chad, Jennifer, and stalwart reporter Jack to their core. She's angry that her father kept leaving her life, furious that her mother was always focused on winning back or moving on from Jack, and has been secretly operating the business with her former stepfather, Peter, through a network of operatives spanning the globe since 2010.
We also learn Ben Weston was in the ISA and was programmed to take her out, but fell in love with her. That's why his murders were OK. André was her partner for a while, but when he turned on her, she dealt with him, and faked her DID because she knew Chad was gullible. Also, Ben fathered Thomas, and Stefan fathered Charlotte (both of whom get shipped to boarding school after a custody battle with Chad and Cat).
An evil Horton, an evil Deveraux, and a Stefano-level evil DiMera all rolled into one - and we can fix a lot of story missteps while we're at it.
(I know it seems less than plausible - but we've got a show where almost everyone was dead at one point or another, so... let's "go with it"!)