Prétear is a 4 volume manga that has one of my favorite romances. It's only 4 books. I will never stop pushing it. Just don't watch the anime. They changed character designs, plot, and the villain.
I second Fruits Basket. You could watch the most recent iteration of the anime. It follows the manga exactly.
There's also My Love Story, I actually haven't watched it yet but apparently it's very wholesome.
Toradora is good as well. The way it handles high school relationships was very believable. I was pleasantly surprised at this one.
I remember liking Say I Love You, but I don't remember much about it.
Kobato. I don't know why this gets no attention. I don't know why I never hear about it. It is, in fact, a romance series. It's CLAMP, so prepare to have your feelings hurt.
And speaking of CLAMP, Chobits is another recommendation. Very wholesome.