What is the climax of 'The Chronicles of Narnia'?

The Chronicles of Narnia is a set of 7 books but I think you're talking about The Lion, The Witch and The Wardrobe. If so:

It's probably when Aslan returns from the dead (although he really wasn't) and rescues the children from the White Witch. Or when they emerge from the wardrobe many years later and are still the same age. But for me it's when Lucy first opened the door of the wardrobe and she finds herself in a new world... Narnia.