So this gets debated a lot, and honestly, the 'right' answer depends on whether you want publication order or in-world chronological order. C.S. Lewis published 'The Lion, the Witch and the Wardrobe' first, and that's where most people start for a good reason. It throws you right into Narnia with the Pevensies, which is a perfect introduction. The world feels discovered, not explained. If you follow publication, you'd go from 'LWW' to 'Prince Caspian', then 'The Voyage of the Dawn Treader', 'The Silver Chair', 'The Horse and His Boy', 'The Magician's Nephew', and finally 'The Last Battle'.
Personally, I'm firmly on Team Publication Order for a first read. Reading 'The Magician's Nephew' first spoils some of the magic of 'LWW'. That moment when Lucy steps into the wardrobe and finds a whole forest is so much more wondrous when you have no idea what Narnia is. Starting with 'Magician's Nephew' gives you all the backstory on the creation of Narnia and the wardrobe's origin, which is fascinating lore but works better as a prequel after you're already invested. The emotional payoff in 'The Last Battle' also hits differently when you've followed the publication journey.
That said, I tried the chronological order on a re-read once. HarperCollins numbers the books that way now, starting with 'The Magician's Nephew'. It's a smoother timeline, moving from creation to end, but it feels less like an adventure and more like a history textbook at the start. The tone jumps around a bit more too. So my solid rec? Start with 'The Lion, the Witch and the Wardrobe'. You can't go wrong following how the author originally released them.