Brax and Charlie. The entire 2011 series was all about them. They were on then off, on again and then off again, on again and then off again, on again and then off again - you get the picture.
Brax and Ricky. More Brax than Ricky. He had such an entitled attitude. After he faked his death for however long it was, he thought he could just go suddenly waltzing back into Ricky's life and she would go running into his arms like nothing had happened. She did get back together with him in the end, but he was always in the background just waiting for her to break up with Nate.
Dean and Ziggy. I firmly believe that they only decided to have Brody suddenly cheat on Ziggy and then decide that Simone was the love of his life because Dean is the type of character that the show promotes and has everyone say what a good guy he is, when the evidence says otherwise. Brody had worked so hard for so long to prove to Ben that he wasn't the person he thought he was, he was genuinely so happy to be with Ziggy and loved her with all his heart, he was so patient, caring and understanding with Ziggy when Maggie had cancer and he saw Maggie as a mother figure. They were a lovely family unit. But come the following year, it was like none of the above with had even happened. Just so they could have Dean and Ziggy as the next supercouple of the show. I'm still really annoyed at how they did this.