I am going old school on this one. My favourite cookbook, The Joy of Cooking, is also one of the oldest in my collection. (And, by "oldest", I mean that I have had it the longest after only The Vegetarian Epicure, which I received when I was 15 years old.)
I will admit that although I have only made a handful of recipes from the book, it is always the first one I turn to when I have questions about, well, almost anything. It's my batter-splashed bible.
My favourite recipes from the book are the chocolate drop cookies and the waffles*. They're not especially fancy, but I have made them again and again with always-perfect results.