Sam Mitchell. This is purely because of those brief yet needless 2009/10 returns, bringing back Danniella Westbrook. Jack Branning getting another Mitchell woman up the duff and that low level feud with Bianca over Ricky carried over from the 90s massacred such a good character who had progressed when Kim Medcalf took over the role. It was a slap in the face of her and Tracy Ann Oberman who made EE one of the must-watch soaps in 2005. Sam's feud with Chrissie, her desperate fight to clear her name, and that memorable episode where she was drunk and dug out Den's body...all of it was undermined by Santer's (then Kirkwood's) ill thought out plan to bring Westbrook back for those short, painful stints.
She is the one Mitchell for whom there is no interest in. Such is the extent to which they hurt the character, just for some mawkish nostalgia. Medcalf could've credibly returned under DTC this year or last if Sam wasn't brought back in 2009. That way she'd have been out of the country for years and wouldn't have been involved with Jack.