You can't go past boxing as the sport requiring the greatest all round levels of fitness. Aerobic endurance, strength, power, speed, agility and flexibility are all extremely important for 
success in boxing. I would make sure I was extremely fit too if I had to get in a ring with someone who wanted to bash me up.

• "Carbohydrate loading for 3 or 4 days before an event can help top up your glycogen stores,” says sports dietitian Joy Dubost, PhD.
• Eat a diet that gets about 70% of its calories from carbohydrates, including breads, cereals, pasta, fruit, and vegetables, to achieve maximum carbohydrate storage.
• On the day of a big event, eat your last meal 3 to 4 hours before exercising, to give your stomach time to empty.
• Avoid eating sugary or starchy foods within 30 minutes of starting an activity; they can speed up dehydration.
• Replenish carbs, minerals, and water during long exercise sessions. Eat a snack and drink fluid every 15 to 20 minutes. Refined carbohydrates (with sugar or flour) pass quickly into the bloodstream, where they fuel working muscles. Many athletes prefer sports bars, sports drinks, or gels, since they're so convenient. But fruit and fruit juice are also excellent choices.
• Reload on carbohydrates after intensive exercise, too. "Since you don't need quick energy, it's best to choose less refined carbohydrates" such as a whole-grain bagel or carrot sticks, which provide both carbohydrates and a rich array of nutrients, Dubost says.

• Know what you need. The average person needs 1.2 to 1.4 grams of protein per kilogram of body weight a day. That's about 88 grams of protein for a 150-pound person. A strength athlete may need up to 1.7 grams per kilogram of body weight. That's about 150 grams of protein for a 200-pound athlete.
• Favor foods. Getting too much protein can put a strain on your kidneys. Instead of protein supplements, eat high-quality protein, such as lean meats, fish, poultry, nuts, beans, eggs, or milk.
• Drink up. "Milk is one of the best foods for recovery after an event, because it provides a good balance of protein and carbohydrates," Dubost says. Milk also has both casein and whey protein. The combination may be particularly helpful for athletes. Research shows that whey protein is absorbed quickly, which can help speed recovery immediately after an event. Casein is digested more slowly, helping to ensure long-term recovery of muscle after a grueling event. Milk also has calcium, which is important for maintaining strong bones.