Seth and Summer
Seth always had a crush on Summer but she just thought he was a geek and didn't think anything about a relationship. When Ryan came to Newport, Marissa started dating Ryan and Summer started to develop a crush on Seth but he's dating Anna Stern and Summer becomes jealous.
After a while Seth decides he really loves Summer and decides to end his relationship with Anna to pursue a relationship with Summer and they both lose their virginity to each other. Summer and Seth broke up quite a few times for numerous reasons including Seth running away and Summer moving on and starting a relationship with Zach, Summer thinking Seth was seeing Anna again when really she was just trying to help him get into college to be near Summer.
In the end they get back together and end up married in the last episode.