I am a bibliovore.
I read at least a book a week and have done so for years. As such, it would be hard to choose any single book as "best".
The book I have read the most often would be
Alice's Adventures in Wonderland, the first "real" book I read as a kid and the book I go to ground with when all else screws up. The world may fall apart, but Alice will still be there, sitting by the bank of the river, trying to understand her sister's reading habits and waiting for a rabbit.
Then there is
Siddartha, which has the distinction of being the book I have given away the most often in my life. I think I have given that book to something like 15, maybe 18 different people. It is a needful book.
The Lord of the Rings is great, but there were so many other fantasy books before it in my life that paved the way. Ultimately, I think I would have to start with either a collection of Poe short stories or
20,000 Leagues Under The Sea on the sci-fi/fantasy/horror count, then
The Three Musketeers and a whole rack full of Arthurian works, especially the works of Chretien de Troyes.
Ah, so many books, such large opthomologist bills...
