If you had to pick one, diet or exercise, which should you focus on? The answer is that it depends what your goals are.
Living a long, healthy life
Exercise, especially as people age, can stave off illness and muscle deterioration. As a result it is more important than diet to elongating lifespan. In fact, even overweight or obese study participants benefit greatly from regular exercise, more so than from intensive weight-loss efforts that involve other factors like dieting. 
Exercise alone will not lead to pure weight loss because muscle weighs more than fat. Yet although exercise will lead to slower weight loss, it targets body fat. For example, a recent study of people coming out of the Biggest Loser competition found that those who maintained just their diets lost 34 kg on average, with 65% of the weight loss coming from body fat. Meanwhile, people who only stuck to the exercise routine lost just 27kg, with 102% from fat (it’s greater than 100% because of the lean muscle mass they gained).  However, I could not find any studies that broke down what % of weight loss comes from eating versus exercising if doing both. So I’m not sure where the often pinned adage that “abs are made in the kitchen” comes from.
The kitchen or the gym?
If you’re primary aim is an energy boost or quick weight loss regardless of muscle mass, then dieting is more effective. However, if you want to live longer or have a healthy, strong body, then you should maximize exercise. And keep in mind that you need to be energized with good food to exercise properly. So in my book, eating well goes hand in hand with exercising, and both are essential to staying fit.