I like The Man Who Was Thursday. Quality stuff.
Chesterton's easily becoming one of my favorite authors. So far I've read The Man Who Knew Too Much, The Club of Queer Trades, and a couple of his Father Brown collections.
What I like about him is that he manages to surprise me. I rarely guess the outcome of his stories before the end.
And the characters! Horne Fischer is easily one of my favorite sleuths of all time... so smart. So cynical. Such an interesting contrast against the Sherlock Holmes types.
A lot of his stuff is available at places like Project Gutenberg and Black Mask, if anybody's interested.