I wonder where The Adventures of Tom Sawyer should be?

I say Gryffindor. Tom is obviously a Gryffindor, and though the tone of the book is somewhat mocking, he is nevertheless clearly the hero and triumphs in the end. (As far as I remember at least; I haven't read it since I was a child).

Yet Twain's ouvre spans all Houses: The Adventures of Huckleberry Finn is Hufflepuff, A Connecticut Yankee in King Arthur's Court is Ravenclaw, and The Man That Corrupted Hadleyburg is Slytherin.