I would have to say "Jane Eyre". I first read it in 5th grade or so; it was the longest book I'd ever read. I loved the story and learned lots of new words from it, that I didn't hear in rural Tennessee! It's the one book that I've ALWAYS had to own a copy of, and go back every few years and reread.
Of course, I love lots of other books too. When we put our Charleston SC house on the market, the realtor walked through and said "get rid of all those books and bookcases". I thought she was crazy, but I cried and did it. Went from nine big bookcases to two; just couldn't get rid of everything. Maybe that's why the house took two years to sell!