Not the fanciest one, not the love-it-now and making every recipe in it but the old faithful. It's not fashionable but it's the most useful one you own. The one you return to time and again and is probably covered in splotches from the very recipes inside it. Be honest!
Mine is an old Nigel Slater, Real Food. I'm genuinely surprised it's that one because I'm not a big fan of Nigel and don't like his other books, but it's the most well-worn and loved book I have. His not-at-all authentic Tom Yum soup is what we all crave when we're ill! I suppose the big appeal is most of these recipes are easy, tasty and most of the family love them. And all standard ingredients!