"The Last Battle"
I found it very hard to read since most of the story is quite dark, but that's why I found it especially appealing. It wasn't a typical mushy predictable fantasy story; it actually invoked emotions and left me awed. This was one of the two Narnia books I read in one day; I just couldn’t put it down!
The ending is debateable but I found it to be a happy ending. I would have hated if the children had to go back to earth and Lewis ended it with "and maybe, one day, you'll find Narnia too", which is what I expected.
Although this is, IMO, the best book I doubt I will reread it again for a long time. I enjoy reliving the innocence of the other books in the series and I must admit that reading ‘The Last Battle’ made me depressed since it was the end of Narnia and a wonderful series.
2nd favourite: Prince Caspian
3rd: Silver Chair