Most nutritionists recommend a high carb, low fat bar for sustained energy during endurance training and races. That means look for a bar with at leatst 65 percent carbohydrates and no more than 20 percent fat Beyond that, it all comes down to taste, convenience and personal preference.

Eat an energy bar about 30-45 minutes before a race and drink 12-16 ounces of fluid along with it to help digestion. During a long (over 90 minute) event, you can take a bite of a bar at 15-20 minute intervals. This, combined with a sport drink during the race will give you a constant supply of energy during the race. At the end of the race you can eat one to regain the lost energy but nothing beats real food like bananas and pretzels to regain the lost vitamins and minerals

References:
1: J. Strength Cond. 10: 1996.