It's so hard choosing favorites! Here are some that left such good impressions on me that I am now buying them for my daughter.
Nancy Drew
A Wrinkle in Time
Number the Stars
The Giver
A Little Princess
A Secret Garden
Ginger Pye
the Dr. Doolittle books
Mr. Popper's Penguins
A Little Princess
Harry Potter (started in middle school and grew up with Harry!)