I was asked a difficult question the other day. What, my interlocutor  wanted to know, was my favourite book of all time?  Did I describe the  question as difficult? Belay that: it is actually impossible to answer.  Other people - and you, dear reader, may well be among that happy band -  might have no problem coming up with an answer.  I am not so fortunate.   For a start, there is no way I can remember all the books I have read  or that have been read to me.  Yes, I did write "read to me".  Well, the  question did say "all time" so I reckon it must cover the books that  were read to me when I was too young to read them for myself.
The  one book I do remember having read to me was Robin Hood.  It was fully  illustrated with colour pictures on just about every page but what I  remember most was my father having great difficulty deciding whether the  word "bow" was pronounced to rhyme with "how" or "hoe".  Later, I  thoroughly enjoyed the Secret Seven and Famous Five books by Enid Blyton  as well as all the Swallows and Amazons titles.  All ripping stuff, but  not really among my all-time favourites.
For a book to be  selected as my favourite of all time it would have to be one that I can  and have read time and again, enjoying each reading as much as or more  than those that went before.  There are many, many books that pass that  test simply on account of my age.  As I get ever closer to my allotted  three score and ten (less than a year to go now) I find that I can pick  up a book and start reading only to discover on page 7 (or 23 or in  chapter 3) that I have read it before.  Not that I necessarily recall  all the details of the plot, far less who "dun it".  But that doesn't  automatically mean the book in question can be regarded as a contender  for the title "All-Time Favourite".  For a book to be in that position  it would have to be one I know I have read before, one whose plot I  remember and in which the denouement comes as no surprise.  That still  leaves me with a fair few from which to choose.
On further  consideration, I have decided I just cannot pick out any one book as my  all-time favourite but I can make a short-list of five titles.  I will  also restrict my selection to no more than one title by any author.   There are several titles I considered but which failed, in some cases  very narrowly, to make the cut.  These include 
Great Expectations by  Charles Dickens and - much to my surprise as I don't usually enjoy  fantasy - 
Lord of the Rings (I know - that's three books but I couldn't  separate the three titles).  So what is on my short-list?  I am slightly  surprised that three of the books fit into the genre "war" although, in  fact, two of them are concerned not so much with the action but with  the reaction of men caught up in war; with their courage and cowardice,  their loves and hates, and with what sustains them in almost impossibly  difficult situations.
And, at last, my fab five (in no particular order) - with brief reviews by other readers.
The Cruel Sea  (Nicholas Monsarrat):  "The story of the crew of a newly commissioned  corvette, acting as an escort to merchant convoys during World War II.  The crew is initially mostly inexperienced, from non-naval backgrounds.  The plot focuses on their differing reactions to some of the horrifying  experiences they have as the German U-boats attack their convoys with  increasing success. Some will survive the war, and some won't - but all  of them will be changed by their experiences."
Birdsong (Sebastian  Faulks): The book's hero, a 20-year-old Englishman named Stephen  Wraysford, finds his true love on a trip to Amiens in 1910.  Unfortunately, she's already married, the wife of a wealthy textile  baron. Wrayford convinces her to leave a life of passionless comfort to  be at his side, but things do not turn out according to plan. Wraysford  is haunted by this doomed affair and carries it with him into the  trenches of the war. 
Birdsong derives  most of its power from its descriptions of mud and blood, and  Wraysford's attempt to retain a scrap of humanity while surrounded by  it.
HMS Ulysses (Alastair MacLean): "When I first came across 
HMS Ulysses,  I read it from cover to cover without putting it down - three times in a  row. The story about the captain and crew of the HMS Ulysses, the story  about men driven to the limit and far beyond by terror, cold and  hunger, who somehow kept going because of their love and devotion to one  extraordinary man, was one of the saddest, most capturing and most  compelling stories I've ever read. I could almost feel the crew's  desperation, feel the piercing cold, hear - and be tormented by - the  captain's ripping cough. Not many books have the power to capture me  that way.  I know 
HMS Ulysses almost  by heart by now - but whenever I read it, I still do it from cover to  cover, without putting it down. Once I begin, I just can't let it go  until it's all over."
In Pale Battalions  (Robert Goddard): "The story is about finding and understanding your  identity. A young lady raised indifferently by her grandparents learns  the truth about her parentage in pieces throughout her life. She is  lucky enough to find love with a wonderful man and have a fulfilling  marriage enjoying motherhood but her past remains a mystery for most of  her life. The title refers to a war and indeed World War I and it's  terrible toll plays an important part of the story, [but] I believe that  the root of the book is identity. The story is told through the  perspectives of a few different people in the life of the main  character. The truth only becomes clear at the very last pages. You do  need to have patience. I found it heartbreakingly true and have read it 4  times.  The writing is compared to Daphne DuMaurier but I don't think  that is necessarily true. What is apparent is the way that layers of the  story are similar to an onion. The more layers that are peeled away,  the more story you find."
Under the Greenwood Tree  (Thomas Hardy): "A poignant little novel. It is a tale of a traditional  country community, it's choir, which is under threat, and a romance. The  novel highlights the beginnings of change for such communities, through  the travails of the "Melstock Quire", which is being threatened by the  introduction of a new organ. Meanwhile Dick Dewey pursues schoolmistress  Fancy Day - although he is not her only admirer. There is a gentleness  and warmth to the characters we meet in Melstock, their traditions and  concerns become ours, it is an absolute joy, a real timeless classic."
And there, in the words of others, you have them.