I voted for Willow and Oz, because it's SO heartbreaking to see Willow so destroyed in the episode Wild at heart, at the beginning of the episode Willow is so worry that Oz has something for Varuca but he doesn't say anything to her, the moment when she sees Oz and Varuca locked in the cage naked. It's so heartbreaking to see Willow so sad again when riley saves her life in the streets till the end of the episode when Oz has decided to leave Sunnydale. It broke my heart plus all was good between them in the end of season 3 when they slept together for the 1st time. But i want to put Bangel the 2nd heartbreaking breakup because the scene in the sewers is also heartbreaking. SMG is so good.