10. Barry Allen and Linda Park
Linda Park is the wife of the Wally West iteration of The Flash in the comics, so when the character appeared as a love interest for Barry Allen in the TV series, some eyebrows were raised. Those eyebrows continued to raise even after their relationship as Wally would arrive soon after Linda's departure.
Nevertheless, Barry and Linda's relationship was fun in the beginning. It was nice to see Barry get to have some fun too, especially since he had been through so much in those first few episodes of the series. But all this romance turned out to be was a plot device and that made it considerably less fun.
While Barry was still understandably struggling with his feelings for Iris, problems began to arise in his own relationship when he couldn't tell Linda about his secret life as The Flash. Naturally, this resulted in typical romance tropes and made audiences long for the relationship to end so that Barry could get back to focusing on his role as The Flash and his affection for Iris.
I didn't hate the relationship between Barry and Linda. It just didn't have too much to offer outside of its purpose to make Iris see that she might be developing feelings for her longtime best friend. It was successful in doing so, even if it wasn't one of the show's great romances.