The Adventures of Tom Sawyer - I remember reading this in fourth grade and being engrossed with the story from beginning to end. All those scenes are still vivid in my mind -- tricking his friends to whitewash the fence, playing with a captured bug in Sunday school, attending his own funeral after everyone thinks he's dead. Mark Twain featured the character again in The Adventures of Huckleberry Finn, Tom Sawyer Abroad, and Tom Sawyer: Detective, and even in a handful of unfinished projects: Huck and Tom Among the Indians, Schoolhouse Hill, and Tom Sawyer's Conspiracy.
Encyclopedia Brown: Boy Detective - Leroy Brown earned his "Encyclopedia" nickname by being a brainy kid who solved mysteries. It inspired me to think that young people might, in many ways, be just as smart (or dare we imagine, sometimes smarter) than adults? The stories made me want to read and learn and read some more.
The Hardy Boys - The old television series The Hardy Boys/Nancy Drew Mysteries, starring Parker Stevenson, Shaun Cassidy, and Pamela Sue Martin (later replaced by Janet Louise Johnson) motivated me to hunt down some of the original books that inspired the show, and I wasn't disappointed.
The Three Investigators - More kid detectives getting into dangerous adventures! Notice a trend? These were probably my favorites as I followed the escapades of Jupiter Jones, Pete Crenshaw, and Bob Andrews.
Which ones were your favorites?