Itazura na Kiss

From ShoujoMagic:

High school senior Aihara Kotoko has had a crush on Irie Naoki since freshman year. But she's been discouraged from confessing to him by a few things: he's a member of "Class A," the top ranking class in school, whereas she's in "Class F"; he gets the top score on every exam; and he's so smart, popular, and handsome that he's been class president every year. When Kotoki finally musters up the courage to present him with a love letter, though, Naoki outright refuses it, telling her point blank--with a look of disgust and boredom--that he doesn't like "stupid girls." Poor Kotoko's worst nightmare! Her heart is broken, but then a change in circumstance forces Naoki and Kotoko to be together every day...!?

Itazura na Kiss is a gem to the shoujo demographic, as it is the mother of a lot of cliches found in today's shoujo series. Despite the fact that it is 2 decades old, it still stands out as unique for it has the series follow the main characters well into their life, over a span of 10 years. It was so popular it inspired popular series like Marmalade Boy and Fruits Basket. 

However, the author had an accident while she was moving homes with her son and husband, and passed away a few months later, leaving her popular manga forever uncompleted. It was adapted into a popular Taiwanese drama under the title "It Started With a Kiss", and it even earned sequel. It was also adapted as a short Japanese drama in 1996, and an anime series in 2008, with the anime portraying the author's planned ending.