This is a good question. It caused me to review the year's cookbooker reviews (yikes, looks like I did a lot of eating) and the year's cookbook acquisitions (yikes again!).
My favorite is Moosewood Restaurant Cooks at Home: Fast and Easy Recipes for Every Day. This was published in 1994, and I picked it up (used) for 50 cents at a library sale. I have prepared 15 recipes from it, some very memorable. There are great suggestions for quesadillas that combine unusual flavors, and have been prepared more than once (unusual in my household). Soups, salad, pasta - none of the recipes has been complicated, required special ingredients, or not worked.
I almost nominated our recent challenge cookbook - the pie and pastry bible. I love it because it taught me so much, but it hasn't had the impact on my family as the other book.