Limiting my normal list to ones that are lighter/happy most of the way through (with the exception of Toradora which has some downer points but still a good ending):
Toradora — High school romance/comedy/drama with some of the best character development over the course of the series.
Akagami no Shirayuki-hime — Independent girl in a medieval land finds her prince and decides to work her way up to him.
Nijiiro Days — Multi-perspective slice of high school life with a quartet of potential couples.
Lovely★Complex — High school romantic comedy with about equal parts of both. A tall girl and a short guy find issues with their heights and like to bicker at each other.
Amagami SS — Somewhat silly, somewhat serious high school romances. Instead of taking a harem approach, this one goes with an omnibus format where every girl gets her own happy ending every few episodes.
Mashiro-iro Symphony: The Color of Lovers — Guy ends up in what had formerly been an all-girls school and tries to keep everyone happy.
Kaichou wa Maid-sama! — A hardworking model student's secret job at a maid cafe is discovered by the only guy at school that can best her.