There isn't any particular food that's the best - eat a good variety of quality foods, and eat alot.
If you're clean bulking, here's what I've been using to great effect:
Protein: Chicken, Turkey, Beef (I eat loads of this), Pork, Lamb, Salmon/Mackerel, eggs.
Carbs: Oatmeal, Beans/lentils, Yams/sweet pots, green veggies (spinach, broccoli etc), cottage cheese.
Fats: Flax / Olive Oil, Nuts, Natty Peanut butter, Avacadoes, Olives.
Make sure you're eating higher than maintenance, try to eat as often as possible (I eat every 2 hours), and drink plenty of water.