The Adventures of Tom Sawyer
Although I know this was written to be primarily aimed at children, it does seem rather dated and there are words used that kids of "that age" today wouldn't understand and many of Tom's antics would seem pretty tame (as would his collection of "treasures" such as marbles, a dead rat to swing on a string, and a brass knob - in this age of commercialism, children would marvel that anyone of any age would think of these things as worth having). This also seems a lot more childish than The Adventures of Huckleberry Finn did, although perhaps that's to be expected as this was written first and the hero is younger here.
Overall, I think I liked this slightly better than The Adventures of Huckleberry Finn but there's not much in it. It's was entertaining enough, but not one I think I'll pick up in paperback.