Chesterton is a considerably harder read for modern readers than CS Lewis, but well worth the trouble. For starters I would recommend any of the Father Brown books, which have no particular order that need be maintained.
The Man Who Was Thursday is a fascinating and thought provoking book, but Very deep and a bit inaccessible. You might want to start with Father Brown (the mysteries come several to a book and may be read in one sitting) before biting off Thursday.
I have not read any of his non-fiction, but from the selected quotes I've come accross I have no doubt that it would be well worth doing.

Bujold is easily in my top-5 modern sci-fi authors. Her Vorkosigan series is well worth reading purely on the strength of its protagonist Miles Vorkosigan, especially the first 2-3 starting with "The Warrior's Apprentice." Some of the middle books ("Memory", "Mirror Dance," etc.) get very dark, but the last few "Komar," "Civil Campaign," "Diplomatic Immunity" aren't quite as depressing / disturbing. The short story "Mountains of Mourning" (included in a few of the omnibus editions) is spectacular, and stands alone quite nicely. The best books combine strong character-driven plots with a compelling, believable universe, laugh-out-loud situations (her favorite and best shtick is including italicized comments from her characters' inner voices) and good elements of mystery and even romance.
"Shards of Honor" and "Barrayar" take place in the same universe and are set earlier than "Apprentice," (they center on Miles' parents' backstory) but I don't recommend starting there for the same reason I don't recommend reading "The Magician's Nephew" before "The Lion, The Witch, and the Wardrobe." You need to know why you *care* about Aral and Cordelia, just like you need to know why you care about Aslan.

Her fantasy frankly isn't quite as good: it lacks characters quite as compelling as Miles, although really only in comparrison. There's nothing Wrong with the fantasy, it simply isn't Vorkosigan. :)

Card is among the most consistent and excellent writers of sci-fi/fantasy currently writing. I love the Ender Wiggin series, but heartily recommend reading it in the order published. You'll be totally out of your depth otherwise.
I also enjoyed the Seventh Son series, although it faltered in the last book or two.
Some of his off-genre books are a little odd, but the only thing of his I've read that I didn't enjoy was "Homebody." Well, "Folk of the Fringe" is a bit odd too. :}
His short fiction is good, and I disagree that it wouldn't be an appropriate place to start. If you're the sort who is put off by a big long series like Ender, then by all means start with the short stories to see if you like his style.

I also enjoy his Biblically-based fiction, even if I don't agree with every liberty he took in the stories.

All in all, you're unlikely to go wrong with Card.

Stephen Lawhead is a startlingly inconsistent author. His best work is among my favorite books ever read. His worst work I can barely get through. I strongly recommend reading the first three books of the Pendragon series ("Taliesin," "Merlin," and "Arthur"), and reading the rest at your own risk. They're not his worst, but they do go downhill from the triumph of "Merlin" and "Taliesin." This is hands down my favorite retelling of the Arthurian myth.
Likewise, "Byzantium" was absolutely amazing.
"Patrick" was firmly in the middle, and what I've read of "Albion" is average too.
But avoid the "Dragon King" series at all costs (if you are over 12 years old), and likewise "Dream thief."