7. Dinah Drake and Vincent Sobel
Dinah Drake and Vincent Sobel's relationship on Arrow season 6 wasn't awful, it was just that it was never portrayed as anything other than a plot device, which made it hard to root (or even care) for it.
Vincent was Dinah's former partner, who she thought perished during the Particle Accelerator explosion that also gave her the Canary Cry. That was the original plan, and then Arrow decided to retroactively change that so that the show could reveal that Vigilante, a mysterious wild card whose identity was still unknown after a full season, was actually Vinnie all along.
The reveal fell completely flat from the beginning because he was now played by a different actor (and thus we didn't recognize him), but even the revival of their relationship didn't change that. It was just there to force conflict between Dinah and Oliver so that it could further the infuriating Old Team Arrow vs. New Team Arrow arc that season 6 was committed to.
Dinah was still a new and underdeveloped character at the time and the Vince reveal ruined Vigilante, so audiences just didn't care about the relationship. It had no substance and only existed to play a part in one of the show's most divisive storylines. That's no basis for a strong romance.