Lets see? Martha was in four episodes as a Bajoran and we can only assume that no one told the actress that she was playing a Cardassian spy and not a Bajoran troublemaker, although stealing the soup from Neelix in Parallax and pushing for B'Elanna to steal that transporter technology from those aliens from Prime factors, it's obvious at the heart of the matter she was a bad egg.
Seska only had one episode as a Cardassian in season one and wasn't seen again till season 2 where she also appeared in 5 episodes. (Followed by two episodes in season 3 and one episode in seaon 7.) So yeah, the bulk of her screen time was divided equally between seasons one and two, and in no way do any of the villains in this poll, except kullah, compare to Seska if she was even only that girl who steals soup from the hedgehog.
How come Caretaker isn't on the list? He raped the crew on purpose and before that killed a 1/4 of Janeway and Chakotay's crews because he was careless bringing them to the delta quadrant after he spent a thousand years infantalizing and boxing the Ocampan Society after he blanched their world? He's not a nice old guy with a banjo.