Triss, dammit. I was going to post a video about why she's great, but literally every result on Youtube is sex scenes. And this poll will be skewed because most people haven't heard of Yennefer if they're only familiar with the games.
Look, I get Yennefer and Geralt's relationship. They can't resist each other. I've had situations where I know I should stay away from someone repeatedly and do not, and it never ends well. Hell, when I saw Yen in that first room, I was just super excited that she was finally in the game. But she's immediately giving Geralt a hard time and putting god knows what up in his regions. Remember at the beginning of Witcher 2 when Triss and Geralt start the game on good terms? And continue to get along throughout the game? That's the kind of stability Geralt needs.