Chesterton is arguably my favourite author, though it's difficult to compare him to say, Tolkien or Rothfuss as I enjoy them for vastly different reasons.
That said, there's a lot more out there worth reading than just TMWWT and Father Brown. My top three recommendations would be
Manalive,
Tales of the Longbow, and
The Poet and the Lunatics.
The Napoleon of Notting Hill gets recommended a lot, but I wasn't impressed as much by it as by some of his other works (same with TMWWT). Four Faultless Felons and Club of Queer Trades were also good reads, though they didn't manage to rank as highly as the three above. Really the same can be said for most of his stuff, it's worth reading but it doesn't all stand out as exceptional.
Several more books/short story collections are listed here:
http://www.cse.dmu.ac.uk/~mward/gkc/books/#FICTIONAs well, look at his non-fiction essays, which I would also recommend, as he keeps up the same entertaining, and thought-provoking, style as in his fiction. Of course, I also agree with most of his basic philosophies, so YMMV on his non-fic.
And since that site's organization leaves something to be desired:
"Novels" (usually collections of short stories centering around a single character and originally published as serials, though some are proper novels)The Napoleon of Notting Hill (1904)
The Club of Queer Trades (1905)
The Man Who Was Thursday (1908)
The Ball and the Cross (1909)
Manalive (1912)
The Flying Inn (1914)
The Man Who Knew Too Much (1922)
The Trees of Pride (1922)
Tales of the Long Bow (1925)
The Return of Don Quixote (1926)
The Poet and the Lunatics (1929)
Four Faultless Felons (1930)
The Paradoxes of Mr. Pond (1937)
The Complete Father Brown, which includes:The Innocence of Father Brown
The Wisdom of Father Brown
The Incredulity of Father Brown
The Secret of Father Brown
The Scandal of Father Brown
I left out the poetry, plays, and individual short stories.
