During his lifetime G. K. Chesterton published six novels: The Napoleon of Notting Hill, The Man Who Was Thursday, The Ball and the Cross, Manalive, The Flying Inn, and The Return of Don Quixote. Years later, long after Chesterton left this world, the manuscript of an unpublished novel written when he was just 19 was discovered: Basil Howe: A Story Young Love.
Of the published novels I have read the first five. I do have a copy of The Return of Don Quixote, and will likely give it a read at some point.
As for the previously unpublished novel, first, the name was just one given to it after it was discovered. We don't know what Chesterton would have given it if he had chosen to try to publish it. It is also hard to find, so there's a good chance I never will get a chance to read it.
Of the novels I have read, I liked Napoleon the best, despite the fact that most critics and readers consider Thursday his best novel. I rank it my second favorite. Ironically on the Chesterton blog to which I contribute I am "Thursday."
My Ranking:
The Napoleon of Notting Hill,
The Man Who Was Thursday
Manalive
The Ball and the Cross
The Flying Inn,
I've also read all the Father Brown stories, but there are a number of his short story collections to read. Indeed, my reading list for this year includes The Poet and the Lunatics.
Given how much Chesterton wrote, I could spend the rest of my life reading just his published essays, poems, plays, and fiction, and probably not get to them all.