For me, it is a tie between Karlach and Halsin.
Karlach because its character is super backward. Sorry but I just can't take seriously that somebody who literally went through hell in the most brutal war of the DnD setting is such a lighthearted and naive character. Her fanbase is also very infuriating, you can't discuss about her without some fanatic treating her like a sacred cow that deserve the best and more. Her not even being present in this poll is really telling.
Halsin because he shouldn't have been a companion at all. He got memed to this role by horny people and as a result, he feels shoehorned in this role. Once his duty is fulfilled in act 2, his only function becomes "the bear sex meme" option. Also he may or may not have taken the place of Helia so we didn't get a small race companion.
