Spike and Buffy. They have the most interesting and compelling relationship, and with such great onscreen chemistry. They offer both contrast and reflection to the other in a mix of ways which makes them work very complementarily in any format, through all the different stages of their relationship. As they develop alongside each other across the tv series, their dynamic progresses with them, finally becoming a very strong, mutually supportive relationship. And it still manages to not become static and dull, they have continued to provide rich stories together onwards into the canon comic continuation. With two such strong individual stories their relationship just seems to draw the energy and spark into it, not running out of ways their dynamic together can keep developing, as well as enriching their individual progress too.