10. Eddie Thawne and Iris West
This one was news to me because I actually liked Eddie Thawne's season 1 romance with Iris West, but apparently a lot of The Flash fans did not. Their biggest issues with it were down to the fact that it prevented Iris and Barry Allen from getting together and that it limited her storyline at times. They also didn't love that Eddie was the partner of Iris' father Joe.
Eddie was the charming and slightly arrogant new detective at CCPD, and other shows might have really dove into the arrogance part of that. But as The Flash went on, we quickly found that Eddie was actually a really sweet, loving guy who just wanted to give Iris the best life he could. And then he found out she was destined to marry Barry.
Some fans didn't love that the relationship made Barry act jealous, while others just felt it was a pointless obstacle preventing the great West-Allen from being together. But here's the thing; this was only season 1. West-Allen was always meant to be and nothing was going to prevent that.
Eddie and Iris' relationship was an unexpected part of season 1, and it remains an unexpectedly divisive part of the show's history among fans. Either way, there was no doubting that they loved each other a great deal.