Calorie Counter & Diet Tracker by MyFitnessPal
Seen by many as the ultimate in dieting and food journal apps, is the Calorie Counter & Diet Tracker by MyFitnessPal. It certainly has the largest food database of any food diary app on the iPhone, having 750,000 entries, so pretty much anything you can think of will be in there (even fast foods), making using the app very easy. But there are custom options too, even allowing you store entire recipes and related information. You have access to all the nutritional information you need when you want to lose weight, including calories, fat, protein, carbs, sugar, fiber, and cholesterol. Another big advantage of this app is that it works offline too, and you can backup your info to the website, so you needn't worry about losing any info.
You can track exercise and use set exercise programs on the app to help you lose weight, as well as set and track personal goals.
It's hard to find anything wrong with this app, as users rave about how much weight they've lost tracking their food intake this way, and doctors are even starting to recommend it.