"The best book I've ever read was Atlas Shrugged in 8th grade. Changed my life." Not because I remember anything about it, or believe anything it espouses, or even like it all that much, but because it's a useful filter for obnoxious people in meatspace. If someone I don't know too well asks me what my favorite book is, I say Atlas Shrugged. If they react inappropriately, I'll be cordial and treat them with respect,…
I do the same thing except substitute The Communist Manifesto.
Different religious texts? Especially if you pick one that is not part of the primary religion of the area you are located in.
A Modest Proposal? Probably the worst choice. Most won't recognize it and the few who do will assume satire in your response. (Also, essays aren't books, but who is counting.)
A certain book by Nabokov?
What about someone who seriously considers the question, but then says that they've read many books but don't really have one they consider the best? Too much of a non-answer?