Ongoing is hard, but if it's just that you don't like short ones that's easy.
Cross Game is 50 episodes. Episodes are available legally on
Viz Anime site.
Touch has 101 episodes, one of the highest rated anime shows ever. (Still picked by Japanese mothers last year as the #1 anime they'd like their kids to watch).
Kimagure Orange Road weighed in at 48 episodes.
Maison Ikkoku has 96 episodes, although it is set in college instead of high school. Also highly rated.
All of those are generally considered masterpieces in the romance genre.