Publication order. Hands down. I could be more elaborate, but for all intents and purposes C3PX said almost everything I wanted to say. You keep so much more of the magic that way. The first time I read the series, it was unbelievable how, as I started closing out the series, Lewis tied everything in together so well. The one moment that stuck out for me was learning about what the Wardrobe was made of. It was at that point I really realized the scope of the series, and quite literally got one of the biggest cases of chills I can remember. It was eerie how it all fit together. My kids are definitely going to have them read to them in publication order. And in any case, The Last Battle is last, which is all that really matters.