Who do you think is the meanest, most cold-hearted and merciless bitch ever to be a Claymore? Can be anyone, as long as they were a Claymore at some point, and consideration of their personalities isn't limited only to their time as a Claymore.
For me there are two, in different ways. Roxanne is the overall winner because we are not shown a shred of decency, empathy, compassion, genuine friendliness or any 'positive' emotions from her, nor does she have any reason why we should apologise for her behaviour or accept it, like with Teresa, Priscilla or Ophelia. She is a real sinister, violent maniac.
Also awakened Priss. Freezing cold, arrogant, completely merciless. 'Irene-san, you seem to be missing an arm. Oh...here it is. I'll give it back to you. You really shouldn't leave your possessions lying around like that'. Also 'Oh, it seems everyone is trying to kill me. But most likely....you can't'. hehe so awesome
Well, most (if not all) awakened beings seem to have some kind of sadistic streak, so it's hard to see which among them is the "meanest"...but the part when Priscilla taunts Ilena about her missing arm was pretty epic.
As for warriors - I think Roxanne and Ophelia are probably tied. People tend to be more sympathetic to Ophelia because of her tragic back story, but it's pretty likely Roxanne suffered the same kind of fate. They were both text book sociopaths.
Roxanne is borderline not sociopathic, and Ophelia is sadistic but she's not characterised well enough to say whether she's really a 'sociopath' or not. She would be diagnosable with Antisocial Personality Disorder according to the DSM criteria though.
Over on a defunct psychology form, a professional psych confirmed many of the CM characters (ie. Clare, Isley, Priscilla, Teresa) would IRL be regarded as psychopathic or sociopathic and CM is remarkable in showing accurately the kind of things psychopaths feel.