When I was a kid it was hands down the Chronicles of Prydain, with Narnia and The Dark is Rising tied as a close second. I [i]really[/i] need to get the boxed sets for nostalgia purposes... Oh right, and anything by Katherine Paterson is pure gold I still love just as much today. Bridge to Terabithia, Of Nightingale's That Weep...really can't recommend any of those enough no matter what age you are. Same goes for Cynthia Voigt and her Tillerman Chronicles. Jane Yolen was pretty great as well, loved the Dragon's Blood series. And of course there's Scott O'Dell and Gary Paulsen...and about a billion more, I'll stop now. I have a serious warm fuzzy place in my heart for so much of the stuff I read as a kid though, it all just got inside my head and had a much bigger impact on my life than most of the stuff I read now.:D
I also read like a billion Nancy Drew books but those didn't age well at [i]all[/i]. Read a lot of Anne McCaffrey's stuff in junior high and I still kind of like the idea of Pern but I just can't stomach anything by her as an adult, she's a terrible writer. :(
As an adult there's Lord of the Rings, obviously, and the Ender saga is pretty amazing as well. Favorite standalone book is a tie between Watership Down and To Kill a Mockingbird. Though the Blue Sword and the Hero and the Crown by Robin McKinley were my favorites in highschool.
As far as actual halfway modern stuff goes, the Species Imperative stuff by Julie Czerneda is hella fun.
edit: Couple of good classics are A Tree Grows In Brooklyn and The Good Earth. Poisonwood Bible as well.
...read a lot of Anne McCaffrey's stuff in junior high and I still kind of like the idea of Pern but I just can't stomach anything by her as an adult, she's a terrible writer. :(