Peter's Room is my favourite because the Gondalling bits - which I used to skip because I don't like adventure stuff much - get more and more interesting as they reveal so much about the characters, eg Patrick's exploration of betrayal. And I like very much the way the fantasy starts to weave into the real events. Slight bits of spookiness like Peter's sovereigns actually being farthings are good - effects of light and dark are very strong in this book - and so is the way the gun turns out to be actually loaded even though Peter's checked them.

Top of my list by a long way.