Out of the list, it has to be Darkonda, for the same reasons JustAnotherBlueRanger said, he kidnapped a little child and had no remorse. He even fought against Eclipter in Rangers Leap of Faith. At least Eclipter had honour, most of the time. And as for Zedd, he and Rita babe were used as a comedy element in PRZ/PRT/PRIS. They were evil, but not mean...you know what I mean
But the award has to go to Banshera. She killed/absorbed Vypra, turned against Diabolico and Olympus, sacrificed Loki, and was just plain Power Crazy, or just crazy!! She's one evil and extremely mean bitch!