Am I right in thinking that the character of Diggory in The Magician's Nephew grows up to be the Professor in The Lion,The Witch & The Wardrobe? I read the The Lion... as a child but didn't get round to reading the others.
The Magicians Nephew is my favourite Narnia book, but you must read the Lion the Witch and the Wardrobe first. It explains everything, how the witch gets into Narnia, how the lamppost gets there, brilliant. (Oh, and yes, the little boy does grow up to be the Professor).
the MN is my favourite too, but I don't think you need to read LW&W first to understand it. The other way round really. Digory buries the apple core and the rings and the tree grows and stays there for many years. then one day it blows down in a storm and Digory makes it into a wardrobe.
The Horse and His boy is my next favourite followed by the last battle which is so sad, especially about Susan.