Can anyone tell me what the absolute healthiest food is so I can start strict diet?

Don't start diet. A diet restricted to one food will ultimately fail because no one food contains all the vitamins, minerals and nutrients the body needs. A vegan lifestyle without processed foods containing white sugar, flour and trans-fats, but containing wild fish like salmon and cod, with some egg whites would be a healthy diet. Also minimize saturated fats and get at least 15 minutes of direct sunlight per day.