People will probably think this is lame... In sixth grade I read The Doom Stone.
I don't even remember if it was actually a good book or not, but the creature on the cover along with how vicious it was in the book really scared the shit out of me.
Popular alternatives to
The Doom Stone
His Dark Materials 3-Book Paperback Boxed Set: The Golden Compass; The Subtle Knife; The Amber Spyglass