I would have to pick Midna.
Midna was a rude, obnoxious, devious, manipulating, slave-driving, cold-hearted little prick from the moment you met her. Honestly, I don't care how much she "changes" by the end of the game, I still hate her. I cheered when she left at the end, not even kidding. Not only does she have a complete lack of respect for Link, she basically blackmails him into helping her by "helping" him find his kidnapped friends. I put "helping" in quotation marks because all she basically does is ride around on Link's back or hide in his shadow, occasionally telling him where to go next in a rude manner. Not only that, she teases Link about Ilia on his way to find her in the Lanayru province. The only times she's really helpful is allowing Link to transform into a wolf and warp him to different areas. That doesn't make up for much, though.
Honestly, in my opinion she's the worst partner character the series has ever had. And while she may not be the worst character in the series in general, she's definitely close to the bottom. In my opinion, at least.