Seska. Easily. I loved / hated her from the moment she was introduced, and she made several annoying Kazon episodes more enjoyable for me, because she was so obviously using them, that it made me feel like I wasn't the only one not taking them seriously. She was also a really interesting, strong, smart female villain.
Though I loathed the episode in which she died ("Basics, Part II").
Not because she died, as I respect a series that kills off characters that are developed; I find that far more interesting and realistic than killing off redshirts. But because of that whole random mess where the baby wasn't Chakotay's. Seska was too frakking smart to make a mistake like that. It was a retcon by Jerry Taylor (she admited this in an interview that I read) who didn't want to put up with the story anymore - and couldn't stand the thought of leaving Chakotay's child (by what amounts to rape) to the Kazon. I don't blame her for wanting to be rid of the Kazon. I don't blame her for killing off Seska for some closure to that storyline (though I think she could have written a better ending in which she survived). But to pull something that huge out of nowhere at the last second felt like a disservice to the character; like lazy writing...because that's what it was.