Smallville timeline only please. If we count comics, movies, other series, etc., then Tess wouldn’t stand a chance.
In one corner is Lex. He killed his father, convinced his former best friend’s love interest she was pregnant, tortured Impulse (Flash), and convinced stated murdered father to be evil again after he tried to go clean.
Other corner is Tess. She kidnapped Lois to violate her memories, kidnapped Jor-el which led to his death, helped Doomsday, would potentially bring about the demise of mankind, and literally shoots her assistants in the back.
Plus, there’s all the things I forgot off the top of my head. I know Tess is supposed to be another Lex, but Lex rarely got his hands dirty. Aside from torturing Impulse and killing is father (which I suppose should be done personally if one is gonna do that), he usually lets others do the dirty work. Tess doesn’t seem to have qualms about that.
Note: I’m excluding the Baltar option for fear he’d Ross Perot the votes. d:
I am tempted to put Chloe in the running though… Plus, Lana was teetering on the darkside for a season too (with good reasons), but I think she’ll be ok.