It's been a while since I've read all 7 books, but I have reread The Magician's Nephew several times, it being my favorite of the books, but I've also reread The Lion, the Witch, and the Wardrobe in high school, and later The Horse and His Boy in college. It's interesting to note, as a child, I was bored by The Horse and His Boy. I didn't see how it related to the rest of the story at all; the setting wasn't even in Narnia, but in a kingdom to the south. But that changed when a friend of mine told me that that The Horse and His Boy was his favorite of the series. That surprised me, it was my least favorite, so eventually, I decided to give it another go. After reading it the second time, I can safely say that whatever changes in my life occurred between then and as a child, I can appreciate the story now for what it's worth. 
 I eventually want to reread 4-7. It's been so long, I barely remember what occurs in them. I suppose that's one of the qualities of the series. They can be read and reread over and over, and still be appreciated. :)