This is quite a tough one because there's nobody I've absolutely loathed all the time they were in the TARDIS. At the start, I absolutely could not stand Mickey, but he proved himself in the end, up to a point. I doubt he'd get away with treating Martha like his possession the way he did Rose at the beginning. I also never got the way he felt threatened by Captain Jack but not by the Doctor - although maybe, given the relationship she had with Ten, he knew her better than I gave him credit for.
I tried very hard not to like Leela. Apart from anything else, she wasn't Sarah Jane, but, although I didn't understand it at the time, there was clearly a very severe dose of patriarchy behind going from a companion who wore stripy dungarees to one who wore next to nothing with boots.I liked the character, though and Louise Jamieson is such a good actress. Similarly, much though it is annoying that Liz Shaw was replaced by ditsy blonde Jo Grant, I couldn't help but like Katy Manning - and without her there would have been no Sarah Jane Adventures Death of the Doctor.
I think I will stick with Mickey as being my least favourite, but don't take that for unreserved dislike.