Well Buffy/Spike has already been mentioned multiple times.......so I'm going to go with one of my faves that HASNT been mentioned.
Sean/Emma (Degrassi TNG) They broke up and got back together again multiple times, but you could tell that they were always the happiest when they were TOGETHER. Sean only wanted to be better when he was with Emma, and Emma showed a different side to her, a more compassionate side, when she was with Sean. They were the PERFECT example of opposites attract imo. I was very sad that it ended the way it did on Degrassi, but that was probably because Daniel Clark gave up on being on the show lmfao.