Honestly, there are so many that I find it hard to choose!
I re-discovered reading YA when my dd was a high level reader looking for appropriate material; and fell in love again. I still like the Anne series, and then there was a series called "In Grandma's Attic" that I was surprised to enjoy as much as I did when I was younger. We enjoyed reading Heidi and The Secret Garden together.
And there are some I'd never read that I wish I'd found. Betsy and Tacy, All of a Kind Family, the books by E Nesbit and the Half magic series...
I do appreciate them on a different level as an adult.