Definitely agreed that Morinaga has her own hit and misses, but who doesn't?
Probably the only three I like from her:
Kisses, Sighs, and Cherry-Blossom Pink - The Nana-Hitomi story someone already mentioned is, to me, her best work.
Girl Friends - It's pretty good but definitely not topping my list but dammit if it isn't fun to read every, single, time.
Princess Mirror - This was a surprise one, didn't know it'd have such an enjoyable ending!