i think that the best narnian book is Prince Caspian. i love how the kids realise that they've been gone for a really long time. and i think that Caspian is one of the best characters.
It came to a draw between LWW and The Magician's Nephew, but LWW is the all-time favorite classic. I love the other stories, also, but I must confess I think my least favorite would have to be The Last Battle. I know, I must sound crazy, but it truly was depressing. Yet through all that awful time, it did come to a very beautiful end in many ways. No wonder Lewis received an award for it. Well merited.
For me it's The Voyage of the Dawn Treader. The idea of going on a voyage through mysterious islands to the "utter east" is mind-blowing. Scary at times, but also stirring.