My favorite cookbook at any given moment is usually the one I've purchased most recently.
Joy of Cooking is probably the one I grab most often, as a reference work, to find time and temperature and general guidelines, but I don't think of it as my favorite. In a way, I've kind of gotten away from having a favorite cookbook, as I now have so many, and depending on my research, several may be useful at the same time.
That said, for at least a couple of decades, I had an absolute favorite that became almost exclusive for a while:
The Cookbook of the United Nations (1970). I got this shortly after getting out of college. Even then, I already had about a dozen cookbooks, but once I had the UN recipes, I was swept away in the joy of exploring cuisines from every nation in the United Nations. Some of the food was not so great, but most of the recipes were fabulous. We're talking 1970s, here, when there weren't shelves and shelves of books on the cooking of, say, Afghanistan or Nigeria, but I had recipes from everywhere at my fingertips. I particularly focused on Africa and Latin America, and had great fun.
In 1978,
The Complete Oriental Cookbook came close to catching up with the UN book, as I dove into Indian and S.E. Asian cooking. I also loved my copy of the classic
Italian Regional Cooking by Ada Boni. I'd say that, until the late-1990s, these were the three books I used the most -- but still in that order of preference, UN, Complete Oriental, Italian Regional. (And they look it.) I also loved
La Technique by Jacques Pepin, but I didn't use it nearly as heavily as these three.
The whole time I was using those books to death, I was collecting and using other cookbooks. Now, I have a few hundred, so my one-time favorites are not used very often anymore. I have multiple cookbooks on Africa, rather than just a smattering of African recipes in the UN book. I have a whole shelf of Asian cookbooks, so Complete Oriental seems less complete. Yet there are recipes I've come to love in all three books, so they are still pressed into service on occasion.
Of course, like many who love cookbooks, there are numerous volumes bristling with sticky notes of things I want to try but have not yet gotten to. So I'm now actually trying to steer clear of the old favorites as I try to work my way through the dozens not yet explored.