Ramsay and Joffrey are both equally bad, they are sadistic. Ramsay has a bit more intelligence but lacks tact. It's funny how Martin makes us hate Theon and then have him replaced by someone worse. It's hard to justify Theon's stupidity though, he's motivated by emotion but lacks Tyrion's skill.
The Mountain is also a sadist with a temper, but you view him as more like a tool than a person (more-so than even his brother).
Tywin and Roose are bad, but they are logical. Which makes it hard to hate them, because they act with reason. As much as I hate betrayal, I can understand why Roose did what he did. Tywin is the worst dad ever, and his treatment of Tyrion is inexcusable.
Cersei lacks intelligence, and is malicious, but she at least has a limit to her maliciousness (when Tyrion isn't concerned).
Frey is an idiot who just wants power and never looks ahead.
I think Littlefinger is the worst, he's malicious, intelligent, sadistic, AND he is motivated by one-sided love.